DeadlyCritic's blog

By DeadlyCritic, history, 4 months ago, In English

A. FashionabLee :

Invented by DeadlyCritic.

Brief Solution
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C++ solution

B. AccurateLee :

Invented by DeadlyCritic.

Brief Solution
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C. RationalLee :

Invented by DeadlyCritic and adedalic.

Brief Solution
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Python solution
C++ solution

D. TediousLee :

Invented by DeadlyCritic.

Brief Solution
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Python solution
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Challenge : Try solving problem D for $$$n \le 10^{18}$$$. (no matrix-multiplication)

E. DeadLee :

Invented by DeadlyCritic.

Hints
Brief Solution
Complete Proof
Python solution
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F. BareLee :

Invented by DeadlyCritic and AS.82.

Brief Solution
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4 months ago, # |
  Vote: I like it -83 Vote: I do not like it

I appreciate your effort, but this contest was very mediocre. Problemsetting might not be the job for you guys. It was basically Geometryforces, and I'm not good at geometry. Please, no more geometry problems.

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    4 months ago, # ^ |
      Vote: I like it +14 Vote: I do not like it

    If you know you're bad at geometry, study up on geometry and stop blaming the author for using too many geometry problems.

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      4 months ago, # ^ |
        Vote: I like it +23 Vote: I do not like it

      As I know the round has minimum possible non-zero geometry.

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    4 months ago, # ^ |
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    I'm not good at CP. Please no more CP problems. Only 1+1 problems.

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    4 months ago, # ^ |
      Vote: I like it +4 Vote: I do not like it

    It sounds like "I don't know geometry, dp, binary search so don't make contests with such problems". Moreover, the only task there with geometry is A...

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    4 months ago, # ^ |
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    O looks like a circle. All statements have at least 1 occurrence of the letter o in them. That's the most geometryforces round I've ever seen!

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4 months ago, # |
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faast tutorial..

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    4 months ago, # ^ |
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    Is there anyone who can explain me the second problem. I can't understand it properly.

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      4 months ago, # ^ |
      Rev. 4   Vote: I like it +2 Vote: I do not like it

      In second one you are given a binary string Carry out the following steps ; .

      1.Select a 1 such that there is immediate 0 next to it
      2.Delete either 0 or 1

      . Repeat the above two steps and continuously reducing the string. There will exist a optimal choices on part 2 such that the string is reduced to smallest length possible and if such lengths strings are many you need to output the lexico wise smallest.

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        4 months ago, # ^ |
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        Nice!!! Thanks

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        4 months ago, # ^ |
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        .

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          4 months ago, # ^ |
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          thnx for pointing out .. its the same logic we are just searching for (10) basically

          # Consider the example in Q

          11001101 → 1100101 → 110101 → 10101 → 1101 → 101 → 01

          # Let me explain
          • here we found 10 pattern at 5th index so we removed 1)
          • Again we found 10 at 1st index ( taking in account of index change due to prev step) then we removed 0
          • Again we found 10 at 0th index then we removed 0
          • Again we found 10 at 1st index then we removed 1
          • Again we found 10 at 0th index we removed 1
          • No match found for 10 so terminated
          Note
          • If you take another choice of removing 1 or 0 in any steps above you'll end up with a larger string by length or by lexico.
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            3 months ago, # ^ |
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            Nice explanation :)

            From my point of view! No need to think about lexicographic or not!

            Just need to reduce as much as 0s by using the condition ith char is 1 and (i+1)th char is 0 from the beginning until the last 0s. Then reduce 1s before the last 0s.

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      4 months ago, # ^ |
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      You can't erase any charecter (neither '0' nor '1') from the input string until you get a '1'.So,you must print all '0' from the beginning.If you get '1' then stop.Now starting from this index of '1' find the position of the last '0'.Now there are two condition=>Condition::1)If there is no '0' then you can't get any index to erase satisfying the problem's condition(s[i]='1' and s[i+1]='0').So,you have two print the whole remaining string(as shown in sample input-output:query-1)......Condition::2)If there is at least one '0' then starting from the idex of the first '1' you can erase all the charecters till the last position of '0'. As stated in the problem you have to print the lexicographically smaller string.So,when the string seems to be "10" then you have to erase the '1'. Summery::1)Print all '0' from the beginning till you get '1'. 2)Print the last '0'(if there is '0')then,all the remaining '1'(if there remains '1').

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      4 months ago, # ^ |
      Rev. 2   Vote: I like it -6 Vote: I do not like it

      This is the Video Tutorial for B https://youtu.be/PYPoOCav4Jk

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    4 months ago, # ^ |
    Rev. 3   Vote: I like it +140 Vote: I do not like it

    Video solutions to all problems are available at the end of my screencast, for people who are interested.

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      4 months ago, # ^ |
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      Please keep doing these videos.. they are really helpful.

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      4 months ago, # ^ |
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      One for all!!Now I don't need to watch different video as I get everything in one place!!!

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      4 months ago, # ^ |
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      It's really helpful. Thanks.

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      4 months ago, # ^ |
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      The quality(explanation) of video forced me to press the bell icon..keep doing this brother .Take love.

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      4 months ago, # ^ |
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      Thank you. Please keep doing this.

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    4 months ago, # ^ |
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    A better way to solve B is to find the first occurrence of 1 and the last occurrence of zero, as you can always erase all the elements from the first 1 till the last zero by deleting one of 0 or 1. You can see my submission for reference.Thankss!

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      4 months ago, # ^ |
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      Why did you take fo!=mod && lz!=mod ?

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        4 months ago, # ^ |
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        That is to make sure that cases like this pass: 5 11111 Peace!

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4 months ago, # |
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Really enjoyed the problemset!

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4 months ago, # |
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In pD: I saw some people use max even under mod 1e9 + 7. Why is that correct.

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    4 months ago, # ^ |
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    Because the numbers you take the max of are very close to each other, so if you assume that the dp values are almost random the chance of there being an error in $$$n \leq 2 \cdot 10^6$$$ is pretty small.

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    4 months ago, # ^ |
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    It seems like there is no $$$n \le 2 \cdot 10^6$$$ such that $$$dp[n][0] \equiv 10^9 + 6 \mod (10^9 + 7)$$$, since otherwise the answer for $$$n = 2 \cdot 10^6$$$ will be different

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      4 months ago, # ^ |
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      Will you please elaborate what are you talking about? Because i solved D and did not have any such doubt but i want to know?

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        4 months ago, # ^ |
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        max(9,11)%10=11%10=1 , but max(9%10, 11%10)%10=9. That's why it "looks" possible that if you always keep the answer mod-ed, taking the max might create problems. That's what people tried to prove/disrove above. I hope it helps.

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    4 months ago, # ^ |
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    And I was so afraid to do that!! Wasted an hour and then ended up doing the same thing. Lowkey felt it was going to fail systest.

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4 months ago, # |
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Can someone help in knowing why B was so hard for me? I didn't struggle on C as much as I did in B. Please help me realize what's wrong in my practice and thought process.

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    4 months ago, # ^ |
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    You should first work out a sound algorithm for every question and then you can code! I hope you use pen and paper to help visualize stuff?

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4 months ago, # |
Rev. 2   Vote: I like it +23 Vote: I do not like it

D can be solved with matrix exponential as well. And one can make the constraint to be n <= 1e9 to force using fast matrix power.

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    4 months ago, # ^ |
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    I am unable to understand this point. Could you pls share some resource to digest your point

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    4 months ago, # ^ |
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    how will you handle the case if i%3==0 then dp[i]++

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      4 months ago, # ^ |
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      You can take vector $$$a_k = [x_{3k}, x_{3k+1}, x_{3k+2}, 1]$$$ and derive formula for $$$a_{k + 1}$$$ in terms of $$$a_k$$$

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    4 months ago, # ^ |
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    Is this method same as what is used for finding Fibonacci numbers ??

    I think it can be done only if we have the relation:

    dp[i] = dp[i-2]*2 + dp[i-1]

    But we also have additional constraint:

    for every i divisible by 3, dp[i]+=4

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    4 months ago, # ^ |
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    The best I could get is a $$$10\times 10$$$ matrix (84825762), which might TLE on $$$n\leq 10^{9}$$$ and probably TLE on $$$n\leq 10^{18}$$$, with $$$10^4$$$ testcases. Can it be done with a smaller matrix?

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      4 months ago, # ^ |
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      A $$$4 \times 4$$$ matrix should work, but matrices doesn't help with solving the challenge, the intended solution works in $$$O(1)$$$ for each test case. (considering $$$x^y$$$ to be calculated in $$$O(1)$$$)

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        4 months ago, # ^ |
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        3*3 can do (in the comment)

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          4 months ago, # ^ |
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          Sure, I can't count, my matrix was $$$3 \times 3$$$ as well.

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        4 months ago, # ^ |
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        Yeah it's not for solving the challenge; I just need to work on my matrix expo skills lol

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      4 months ago, # ^ |
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      Can you do it with a 5x5? You need just a linear combination of dp[n-1], dp[n-2], n%3==0, n%3==1, and n%3==2, in order to determine dp[n], right?

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        4 months ago, # ^ |
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        Your method seems cool too!

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        4 months ago, # ^ |
        Rev. 3   Vote: I like it +3 Vote: I do not like it

        Oh, so you cycle the extra addition instead of the whole equation. That's neat! Edit: implemented this here 84829787

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          4 months ago, # ^ |
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          Can you explain what does the matrix represent? How does the values of matrix a are filled?

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          4 months ago, # ^ |
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          Can you explain how do you use the (i%3) condition in your linear relation?

          Sorry if it's a trivial question.

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            4 months ago, # ^ |
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            If you write out the matrix on paper, you'll see that each time you multiply some initial vector by the matrix, the last 3 entries will shift cyclically. You can use that to add a value every 3rd step.

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            4 months ago, # ^ |
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            dp[n]=2*dp[n-2]+dp[n-1]+( n%3>0 ? 4:0); dp[n]%=1000000007;

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    4 months ago, # ^ |
    Rev. 3   Vote: I like it +22 Vote: I do not like it

    T =
    0 1 0 dp[i-1]
    2 1 0 dp[i]
    0 0 1 1

    T3 =
    0 1 0 dp[i-1]
    2 1 1 dp[i]
    0 0 1 1

    T3 * T * T

    Hope this is clear enough :)

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      4 months ago, # ^ |
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      maxwill Using your relation, I came up with a formula for when n%3==0,1,2 respectively, but there seems some mistake which I'm not able to figure out.

      [T_(n-1)]     T^(2*(n-3)/3) * T3^((n-3)/3) * [0]
      [T_(n)  ]=                                   [1]
      [1      ]                                    [1]
      

      The above applies when n%3==0, and similarly I've worked out other two cases, but idk why this starts giving wrong output from 9 onwards. Is there some mistake in this relation? How do I correct it. I've spent hours.

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        4 months ago, # ^ |
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        (ABC)^n != A^n B^n C^n. And your T is ambiguous.

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          4 months ago, # ^ |
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          T_n is the nth term. T and T3 are the matrices {{0,1,0},{2,1,0},{0,0,1}} and {{0,1,0},{2,1,1},{0,0,1}} Final answer of any n will be n*4. I haven't made any mistake in calculating power of the matrix, I've checked that function. I don't know what am I missing x

          UPD: I was doing a blunder in multiplication. Therefore, changed the matrix. Now, rather than using multiple 3X3 matrices, I am using one 4X4 matrix and finally got AC. 84893815.

          I still wonder, how do we solve it using two 3X3 matrices.__

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            4 months ago, # ^ |
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            Your program is doing TTT*...*T3T3T3*x
            While the answer should be T3TT*...*T3TT*x

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    4 months ago, # ^ |
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    That is a force online solution, which would've make it a bit much more harder :)

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      4 months ago, # ^ |
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      I don't think so. Whoever can come up the recurrence AND knows matrix exponentiation will definitely be able to implement it quite quickly. In fact, I tried the mat expo first, only to notice later that it wasn't necessary. A little harder, yes. But still, well within the range, I believe.

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        4 months ago, # ^ |
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        Maybe, but I messed up with recurrence (mod 3) part and didn't think about matrix expo.

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4 months ago, # |
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WOW!! the editorial came fast!!

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4 months ago, # |
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a great contest!

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4 months ago, # |
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what's wrong with my solution?

DIV 2/C

solution link : https://codeforces.com/contest/1369/submission/84811103

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4 months ago, # |
Rev. 2   Vote: I like it +90 Vote: I do not like it

Lee

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4 months ago, # |
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How the fuck is the editorial blog uploaded 5 hours ago when the contest started 2.5 hours ago !!?

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4 months ago, # |
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Did you really expect the contestant to prove C during the contest, or just do a Proof By AC? I mean I got 3 WA trying different greedy algos without proving them and I knew that almost everyone submitted without being completely sure of the proof.

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    4 months ago, # ^ |
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    Lol is there something to prove? It was obvious for me

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    4 months ago, # ^ |
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    I did proof it in $$$2$$$ minutes, but my English sucks so It became so long.

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    4 months ago, # ^ |
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    I think the proof is quite intuitive. The author's explanation is too wordy.

    Notice that

    • We can break down the problem into finding the $$$\sum_i MAX(friend_i) + \sum_i MIN(friend_i)$$$

    • For some large $$$a_i$$$, it is easy to use it to maximize the $$$\sum_i MAX(friend_i)$$$ by giving it to a person with no gifts. This suggests for each person's initial gift, we should be handing out one gifts to each person in decreasing order

    • In order to maximize $$$\sum_i MIN(friend_i)$$$ as well, we notice that $$$MIN(friend_i) == MAX(friend_i)$$$ if $$$w_i = 1$$$. So, we reorder our gifts so people with $$$w_i = 1$$$ get the largest gifts.

    • Because of the way the MIN function works, it' doesn't make sense to give someone many gifts with high $$$a_i$$$, then one gift with low $$$a_i$$$. Similar to last point, it's more efficient to give the better gifts to friends with smaller $$$w_i$$$. This way, we burn less of the good gifts on people. This suggests that we should use a greedy solution to fill the remaining $$$w_i$$$ for each friend and we should fill the friends with smaller $$$w_i$$$ first, with the largest unused $$$a_i$$$.

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      4 months ago, # ^ |
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      This is a garbage proof.

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        4 months ago, # ^ |
        Rev. 2   Vote: I like it +37 Vote: I do not like it

        The purpose of a proof is to convince someone that a solution is correct.

        Can I spend 10 more minutes to come up with a more rigorous proof? Probably. Do I want to spend 10 more minutes to do it during a contest? Probably not.

        And since we're being evaluated on our ability to code and not on our ability to prove our solutions, using non-rigorous proofs is the correct way to go as long as the proofs can convince OURSELVES that a solution is correct.

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          4 months ago, # ^ |
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          This is not an unrigorous proof because it is not anything resembling a proof.

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          4 months ago, # ^ |
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          But don't reply now, I have got no patience.

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          4 months ago, # ^ |
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          In almost every wrong greedy you can come up with something to convince yourself. The purpose of the proof is NOT to convince, the purpose of the proof is to give 100% confidence. Convince seems a weaker word to me, I am usually convinced by every other wrong solution.

          I don't feel good about your proof. For example, why try to maximize MAX for each friend first? Can increasing MINs at the cost of some MAXs be better? Of course, the answer is NO. Simply, because here, the algorithm is indeed correct. But your proof is a bit too handwavy to address many of the critical issues.

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        4 months ago, # ^ |
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        Ok, how about this proof? First, you sort your integer Array from largest to smallest and start distributing to friends from the front. also you sort your friends from smallest to biggest by w.

        the first priority friend is the friend with w = 1. because if you give to that friend, the happiness is boosted twice. After giving the integers to that friends with w=1, the second priority is maxing the MAX of the leftover friends. that way, you dont waste your big numbers between MAX and MIN of the friends. you distribute your numbers one by one to your friends and max their MAXes.

        then you need to start wasting your numbers, the way to do it is starting with friends with small w so you dont waste as much numbers.

        end?

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          4 months ago, # ^ |
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          This is an excellent proof

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          4 months ago, # ^ |
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          There is no point of considering the case when w = 1 , just sort the array of integers in non — ascending order and distribute the maxes among K friends. That way, w = 1 case already got handled. And then continue with your process.

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          2 months ago, # ^ |
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          @Wonsei Thank you so much for the explanation. I was stuck at this the whole day.

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      4 months ago, # ^ |
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      The first bullet point in your proof was the main thing I needed to realize to solve this problem.

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Stucked at B for a while? Thinking what should i remove 1 or 0 in the middle part? Can someone gimme idea?

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    4 months ago, # ^ |
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    The key intuition here is that you cannot change the leading 0s or trailing 1s. Anything in between, however, can be converted into a 0 in some way, so you don't need to remove anything. All you have to do is keep the leading 0s and trailing 1s, and if elements remain in the middle, add a 0 in between.

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    4 months ago, # ^ |
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    the 0 is added is the 0 right before the trailing 1s in original string

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    4 months ago, # ^ |
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    There were just certain observations that

    1.You can't change leading zeros and trailing 1s
    2.Anything in between can be transformed to 1 or 0

    Like *****110010**** — ****10010**** — ****1010**** — ****110**** — ****10**** — ****1**** or — ****0****

    3.But since we need lexico wise smallest we'll take 0
    4. Hence our answer becomes (leading 0s) + 0 + (trailing 1s)
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    4 months ago, # ^ |
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    Check this out for solution to C

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An ideal contest with ideal timing and ideal problems! Liked it!

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Fastest system testing I have ever seen.

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The problems were LoveLee!

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Wow. What an editorial. I honestly felt like I made some problems harder for myself than they actually were LOL

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I will add implementations soon.

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Tutorials are so fast , Thank YOU !

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In problem D I am not getting where I am making mistake Can some one help https://ideone.com/RyTb1E

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4 months ago, # |
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unrated

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4 months ago, # |
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O(N + M) solution for E. We need to make the observation that as long as the degree of a node is not greater than the value of the node, it is always optimal to remove it as late as possible. Then we can run a process similar to topological sort to check if we can remove all edges.

Link: https://codeforces.com/contest/1369/submission/84803734

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
 
using namespace std;
using namespace __gnu_pbds;
 
typedef tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update> ordered_set;
 
#define ll long long
#define ld long double
#define pii pair<int, int>
#define f first
#define s second
#define readl(_s) getline(cin, (_s));
#define boost() cin.tie(0); cin.sync_with_stdio(0)
 
const int MN = 200005;
 
int n, m, cnt[MN], freq[MN], vis[MN], rem[MN];
pii a[MN];
vector<pii> adj[MN];
vector<int> v;
 
int main() {
    boost();
 
    cin >> n >> m;
    for (int i = 1; i <= n; i++) cin >> cnt[i];
    for (int i = 1; i <= m; i++) {
        cin >> a[i].f >> a[i].s;
        freq[a[i].f]++; freq[a[i].s]++;
        adj[a[i].f].push_back({a[i].s, i});
        adj[a[i].s].push_back({a[i].f, i});
    }
    queue<int> q;
    //for (int i = 1; i <= n; i++) printf("%d ", freq[i]);
    for (int i = 1; i <= n; i++) if (freq[i] <= cnt[i]) q.push(i);
    while (q.size()) {
        int cur = q.front(); q.pop();
        vis[cur] = 1;
        for (pii nxt : adj[cur]) {
            if (!rem[nxt.s]) {
                rem[nxt.s] = 1, freq[nxt.f]--; v.push_back(nxt.s);
                if (freq[nxt.f] <= cnt[nxt.f] && !vis[nxt.f]) q.push(nxt.f);
            }
        }
    }
    int mis = 0;
    for (int i = 1; i <= n; i++) if (!vis[i]) mis++;
    if (mis) return 0 * printf("DEAD\n");
    printf("ALIVE\n");
    for (int i = v.size() - 1; i >= 0; i--) printf("%d ", v[i]);
 
    return 0;
}
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    4 months ago, # ^ |
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    Nice. :)

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    4 months ago, # ^ |
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    this is clean

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    4 months ago, # ^ |
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    Thanks for the solution. I have a doubt though. instead of checking !rem[nxt.s] why can't we just use !vis[nxt.f]. I tried it didn't work but couldn't understand why.

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      4 months ago, # ^ |
      Rev. 2   Vote: I like it 0 Vote: I do not like it

      Hopefully you see why with this case:

      2 1
      2 2
      1 2
      

      Edit: Sorry, this case does not disprove your point due to the weird way I checked the vis array. However, you must realize that edges should still be removed even if both nodes it connects are visited :)

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        4 months ago, # ^ |
          Vote: I like it 0 Vote: I do not like it

        I got the case See the 5th test case. The logic that we need not remove an edge if both nodes are visited is perfectly alright. But what is happening is if we check only that we will be repeating operations for the same edges because queue may contain multiple entries of the same node. suppose you have three nodes between 1 and 2. Then if you start from 1, you will push 2 into the queue 3 times. Now when you iterate for 2 if you don't check the edges then you will loop 3 times for each edge. Hence you get an error. Btw, thanks for looking into it.

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          4 months ago, # ^ |
          Rev. 2   Vote: I like it +6 Vote: I do not like it

          Precisely. The duplicate entries are caused by the abnormal placement of the vis[cur] = 1 line, which in turn causes the error you mentioned. Please see the more accurate implementation here, where the case I mentioned would effectively break if you checked for !vis[nxt.s] instead.

          I guess the lesson to be learned here is to always implement as precisely as possible, otherwise debugging may become a lot harder :D

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4 months ago, # |
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Thanks for the great quality questions! I really enjoyed solving it :) The difficulty gradient was so good for me ;)

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4 months ago, # |
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Fast system testing as well as tutorial. Seemed that it was programmed that tutorial will be uploaded automatically when the system testing is over. :D

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4 months ago, # |
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Extremely well written tutorial,super easy to get

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4 months ago, # |
  Vote: I like it +14 Vote: I do not like it

Special thanks for hints in problem E, actualLee they are better than the complete solution.

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4 months ago, # |
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Really really cool problemset. Thoroughly enjoyed. Kudos!

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4 months ago, # |
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ezgif.com-add-textccba5ace14bf767d.gif.

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    while(t-- > 0) {
        int n = sc.nextInt();
        String s = sc.next();
        int left = 0;
        int right = n - 1;
        while(left < n) {
            if(s.charAt(left) == '0') {
                left++;
            }
            else {
                break;
            }
        }

        while(right >= 0) {
            if(s.charAt(right) == '1') {
                right--;
            }
            else {
                break;
            }
        }
        String result = "";

        for(int i = 0; i < left; i++) {
            result += "0";
        }
        if(right + 1 != left) {
            result += "0";
        }
        for(int i = 0; i < n - right - 1; i++) {
            result += "1";
        }
        System.out.println(result);
    }

B. TLE on test case 9. How to optimise it?

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    4 months ago, # ^ |
      Vote: I like it +8 Vote: I do not like it

    Try using StringBuilder instead of += for making large strings.

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    4 months ago, # ^ |
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    Also, use an object new BufferedReader(new InputStreamReader(System.in)) to read the input instead of Scanner if there's lots of it. Also, use new PrintWriter(System.out) if outputing a lot of data.

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    4 months ago, # ^ |
      Vote: I like it -15 Vote: I do not like it

    B. Accuratelee Explanation Video >>> https://www.youtube.com/watch?v=EoJetsWa9k0 Hope it helps :)

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4 months ago, # |
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Can someone please tell me why am i getting WA on pretest 2 in C : Link to code

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4 months ago, # |
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nice contest.

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4 months ago, # |
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An easy greedy strategy for B:

Iterate over the string from the end, and for every "10":

  • Delete all consecutive 1s before it
  • Delete all consecutive 0s after it
  • Delete the "1" from the "10"

Solution here.

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4 months ago, # |
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rated or not?

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4 months ago, # |
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Nice problemset!

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4 months ago, # |
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Fastest system testing as I have seen

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4 months ago, # |
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You need to multiply (i % 3 == 0 ? 1 : 0) part in D's brief solution by 4

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    4 months ago, # ^ |
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    cause as you can see after every 3 there is one extra claw that is getting added to the tree like in the case of N being 6 answer should be ans[5]+ans[4]*2 plus that extra claw . btw I don't own this image

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4 months ago, # |
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Can anybody explain how we form the graph in problem D? I read the problem statement multiple times but couldn't figure out how they made the Level 4 graph. :(

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    4 months ago, # ^ |
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    1. Add one child to nodes with 0 children
    2. Add 2 children to those nodes which already have one child
    3. And if the node has already more than 1 child, we don't need to add any more child to it.

    So,3rd level graph is given int the question

    Nodes with no children -> 3,5,4 Add 1 child to these(8,10,9 respectively) , Nodes with 1 child -> 2 Add 2 children to it(7,6) ,

    No need to add children to 1 as it already has more than 2 children

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4 months ago, # |
  Vote: I like it +36 Vote: I do not like it

Man. Div. 2 A's are getting harder and harder these days. Most people will probably intuit the right answer, but I usually prove before submitting, and some recent Div. 2 A's have very non-trivial proofs.

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4 months ago, # |
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I solved D using normal 2D dp,

dp[i][1] = dp[i-1][0] + dp[i-1][0] + dp[i-1][1] and

dp[i][0] = max(dp[i-1][0],dp[i-1][1]) + max(dp[i-1][0],dp[i-1][1]) + max(dp[i-2][0],dp[i-2][1])

I created a DS to store quotient and remainder to compare modulo values

My solution: Link

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4 months ago, # |
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Can someone explain why my solution is giving WA on tc2 Link: 84818110 I am unable to identify any loop holes

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4 months ago, # |
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Wow, I made solution to problem D that is different from editorial https://codeforces.com/contest/1369/submission/84820772 I used cnt[i] — count of added vertices in i level, and lol[i] — answer for i level

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4 months ago, # |
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Very Good Contest and Fast Editorial

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4 months ago, # |
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Lee is everywhere :D

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4 months ago, # |
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Here's an easier analysis for A. The exterior angle (diagram below) of a regular polygon of n sides is 360/n
In order for one side to align to the vertical and another side to the horizontal, the sum of the exterior angles of some k consecutive sides must equal 90.

We must have
(360/n) * k = 90
k = 90*n/360 = n/4
So n must be a multiple of 4 for such an integer k to exist.

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    4 months ago, # ^ |
      Vote: I like it +5 Vote: I do not like it

    I came up with the same proof, but it was harder for me to write it in English. Thanks for doing it instead of me. :D

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      4 months ago, # ^ |
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      My pleasure. I wish coming up with solutions quickly was easier for me than writing proofs :D

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4 months ago, # |
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can someone give me a link or explain to me the proof to A as i don't really get the editorial which is my bad :)

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https://codeforces.com/contest/1369/submission/84793979

could someone help me with this submission.dont know why is this wrong, i guess its the same approach as in editorial

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    4 months ago, # ^ |
      Vote: I like it +3 Vote: I do not like it

    Try

    2
    4 2
    4 3 2 1
    3 1
    4 2
    4 3 2 1
    1 3
    

    Should it matter if $$$w = [1, 3]$$$ or $$$w = [3, 1]$$$ ?

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4 months ago, # |
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Great contest with perfect Timing and as well as excellent problem set.

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4 months ago, # |
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Is there a greedy solution for D.TediousLee

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4 months ago, # |
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I have done the exact same thing , why am i getting wrong ans verdict on c qn https://codeforces.com/contest/1369/submission/84794143

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4 months ago, # |
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Feeling Very Sad,this is the first time i have solved 3 problems in div 2. But could not submit C because of the power supply went off for about 30 minutes.

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4 months ago, # |
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So fast tutorial!Thank you Codeforces.

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4 months ago, # |
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DeadlyCritic In problem E, the last sample test case:

INPUT:  
4 10   
2 4 1 4  
3 2  
4 2  
4 1  
3 1  
4 1  
1 3  
3 2  
2 1  
3 1  
2 4  
OUTPUT:  
DEAD  

Suppose, following is the ordering in which Lee's friends eat[u -> v means (u-th friend eats v-th food)]:

9 -> 3
6 -> 1
4 -> 1
8 -> 2
7 -> 2
1 -> 2
10 -> 2
5 -> 4
3 -> 4
2 -> 4

My answer:

ALIVE
9 6 4 8 7 1 10 5 3 2
Why is this ordering wrong?

Please help!

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    4 months ago, # ^ |
    Rev. 2   Vote: I like it +8 Vote: I do not like it

    when friend i goes to kitchen, he tries to eat x_i AND y_i, even the x_i is exists

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      4 months ago, # ^ |
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      so, the 9-th friend eats not just 3, but 1 too

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      4 months ago, # ^ |
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      Oh, Okay! So, if both favourite foods of the friend exist, he/she will eat both of them? You mean this, right?

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        4 months ago, # ^ |
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        It was clearly explained in the statements =(.

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          4 months ago, # ^ |
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          I am sorry but I thought the same too ... Except for that it was a great problemset, looking forward to more contests by you

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            4 months ago, # ^ |
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            We were expecting some people to thing like this, so we spent quite a long time to write the statements as clear as possible, I'm sorry that it was not as good as I expected.

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4 months ago, # |
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i really don't see how the proof for problem D says that we should use i % 3 and that magic while computing the answer?

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    4 months ago, # ^ |
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    Let the answer for RDB of level $$$i$$$ be $$$dp_i$$$ . Also an RDB of level i is connected to two RDB of level $$$i-2$$$ and one RDB of level $$$i-1$$$.

    1. Note that it is beneficial to ignore the root vertex of RDB of level $$$i$$$ if $$$dp_{i-2}$$$ requires the root to be included. Because, at the cost of adding one claw of $$$i-1$$$, we'll lose two claws, one for each RDB of level $$$i-2$$$.

    2. For $$$dp_{i-1}$$$, it doesn't matter whether we include the root or not because either way no extra claw gets added or removed.

    3. Also if both $$$dp_{i-1}$$$ and $$$dp_{i-2}$$$ doesn't require their root to be included to get their respective values, we'll add root of RDB of level $$$i$$$.

    Point 3 gives an idea on whether to include root or not in point 2. That it is if including the root doesn't matter in getting $$$dp_i$$$, we'll always not include it. Because in doing so, we may get a better answer for $$$dp_{i+2}$$$. Say we'll include root for $$$i$$$, then we can't (or rather shouldn't) include root for $$$i+1$$$ and $$$i+2$$$, but this implies we'll include root for $$$i+3$$$. In the question, we have to include root for $$$i = 3$$$, so we have to include root for $$$i=6,9,12,..$$$ as well. That is where that $$$i$$$ % $$$3$$$ comes into play.

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      4 months ago, # ^ |
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      Great explanation, thank you, idk how didn't that cross my mind.

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4 months ago, # |
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Is there any way to see full test case, my code failed system testing in B but I can't figure out on which test case: https://codeforces.com/contest/1369/submission/84767454

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4 months ago, # |
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Why this doesn't work for D? I find for every level how many more(new) vertices with 3 children were added from previous level and then do dp[i]=dp[i]+dp[i-2]. Solution: https://codeforces.com/contest/1369/submission/84822790

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    4 months ago, # ^ |
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    You are saying you count new vertices but you are doing dp[i][2] = dp[i-1][2]+dp[i-1][1]. So it is getting added cumulatively.Also for ith layer you need to do dpp[i] = dp[i][2]+dpp[i-3]. You can see this after drawing some trees.

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      4 months ago, # ^ |
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      Yea dp is adding cumulatively but dpp stores the added vertices as they come only from dp[i-1][1].

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https://codeforces.com/contest/1369/submission/84819647 Can anyone tell me whats wrong with my C solution. First I sorted the interger lee has in descending order.Then I sorted w array in ascending order. Then I took the case for when w[i]=1. I gave them maximum value.Then I disitributed interger to W[i] one by one. It will be a great help if anyone call tell me my mistake

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4 months ago, # |
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What's wrong with my solution ? Question B ; My solution : https://codeforces.com/contest/1369/submission/84814815

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4 months ago, # |
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Why there are not the codes for the solutions in the editorial?

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    4 months ago, # ^ |
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    I'm working on it, I have a very bad headache right now so I'm pretty slow.

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4 months ago, # |
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In D can someone explain why is 1 added if i is divisible by 3

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    4 months ago, # ^ |
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    Because that gives an AC !

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    4 months ago, # ^ |
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    Because when i%3 u can use the top claw (1;2;3;4)

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      4 months ago, # ^ |
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      Oh damn, can you also explain how did you reach to this in the contest? What was your approach to the question?

      Thank you!

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        4 months ago, # ^ |
          Vote: I like it +12 Vote: I do not like it

        Suppose you used the root for some x. Now, x will be a child of both x+1, and x+2 so, in either of them, you can't use the top claw (because it's child is x and it's root has been used already). Now, for x+3, it's children will be x+1, and x+2 whose roots are free and you can use the top claw in this case. It's a period of 3, just check where it starts.

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        4 months ago, # ^ |
          Vote: I like it +4 Vote: I do not like it

        Here's my thought process in this problem:

        If you look at the case n=4 closely, you'll see that the tree RDB(4) is actually composed of a root node linking to 2 instances of RDB(2) and an instance of RDB(3).

        Now, because of this, we might start to think that a[i]=a[i-1]+2*a[i-2]. This formula holds true for n=5, but doesn't for n=6 if you check by hand. Why is this? It turns out that, again, checking by hand, in the case n=6 we can use the top claw.

        Again, we think about when it is possible to use the top claw. Clearly, when n=4 and n=5 we couldn't use the top claw because we did that when n=3. However, when n=6, we can use the top claw as we did not when n=4 and n=5.

        We see that we can use the top claw in the case n when we didn't in the case n-1 and n-2. Seeing as 3 is the first n to allow this, we conclude that we can use the top claw if n is divisible by 3.

        Therefore we have the above formula: a[i]=a[i-1]+2*a[i-2]+4*(i%3==0).

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          4 months ago, # ^ |
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          This was very helpful. Thanks a lot!

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4 months ago, # |
  Vote: I like it +6 Vote: I do not like it

question B sucked my whole contest.

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4 months ago, # |
Rev. 3   Vote: I like it -7 Vote: I do not like it

hi guys try these video solutions ->

Problem A

Problem B

Problem C

Problem D Tried to explain graphically how DP is working :)

Hope it Helps.. :)

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4 months ago, # |
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I accidentally used Ideone on public settings and someone copied and submitted the same code int this contest and now i received a system email regarding the penalty for this. Can someone guide me as to what should i do. This is the link of my submissions https://ideone.com/qDpu3b

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4 months ago, # |
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simple and clear

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4 months ago, # |
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Nice way of writing editorial...!!! DeadlyCritic

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4 months ago, # |
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Implementations are out, sorry Python solution for D is missing yet.

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    4 months ago, # ^ |
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    Just volunteering:)

    python code for D
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4 months ago, # |
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For pB I tried to use a "more greedy" approach, which consisted on going backwards on the string and once I had a zero I would starting deleting ones until the last one. Then I looped again though the string, starting from the first One and deleting all zeros that followed, except for the last, which was kept and the One deleted. It ended up being pretty messy, but it worked, even tough I didn't had time to submit it during the contest because of some stupid bugs. Here is my solution, you won't have a good time looking at code(it is pretty messy as I said), but here it is: 84823891

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4 months ago, # |
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https://codeforces.com/contest/1369/submission/84825830 is there anyone who can tell me why I am getting Memory limit exceeded on test 2

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4 months ago, # |
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for problem D,someone please tell me why this dp relation is wrong,i made two cases if we color root and if not, v[i]=max(((2*v[i-2])+v[i-1]),((4*v[i-4])+4*v[i-3]+v[i-2]+4))

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4 months ago, # |
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Can someone help me figure out why am I getting a WA in Problem C.

Link to my submission

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4 months ago, # |
  Vote: I like it +1 Vote: I do not like it

Even after giving so many contests, I can't solve div2 C problems. I can solve them in upsolving but I can't solve them during contests. Can anyone please help me to overcome my this problem, please even single advice will help me. Btw here is the link to my upsolved div 2 C problem. I am expecting help from the community. Help me.

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4 months ago, # |
Rev. 3   Vote: I like it +3 Vote: I do not like it

What's wrong with my solution? 84826635

I use a 3d dp to keep a count of vertices with 0, 1 and 3(the outer ones, as they will only contribute to the next state) children. Then I say that:

dp[0][i] = (2*dp[1][i-1]%MOD+dp[0][i-1]%MOD)%MOD;
dp[1][i] = (dp[0][i-1])%MOD;
dp[3][i] = (dp[1][i-1])%MOD;

I fill the dp till i = 2000002 using the above relation. And finally output (dp[3][n]*4)%MOD

There is some mistake in this logic. But I'm not able to figure out where and how to fix it?

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    4 months ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    you can't take all claw. In n'th level if you have total m claw then you can take less then m claw. Because some claw's overlap with another claw. see my solution.

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      4 months ago, # ^ |
      Rev. 2   Vote: I like it +3 Vote: I do not like it

      Can you elucidate it a bit, please? I don't understand what is the use of calculating dp[i][3], in your solution? And how exactly are you working out states in sol[]?

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        4 months ago, # ^ |
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        okay.in my solution here,

        dp[i][0] is the number of nodes which has 0 child in i'th level, dp[i][1] is the number of nodes which has 1 child in i'th level, dp[i][3] is the number of nodes which has 3 child in i'th level, at the same time dp[i][3] denotes total number of claws.

        in sol[] array I calculate the solution for n'th level in sol[n].

        in my solution,

        sol[n]= sol[n-3] + number of new claw added in n'th level * 4 .

        ** claws which were added newly in n-1 and n-2 th claw are overlapped.So i didn't count them.

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      4 months ago, # ^ |
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      Can you provide a test case where this method would fail..?

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4 months ago, # |
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B. Accuratelee Explanation Video >>> https://www.youtube.com/watch?v=EoJetsWa9k0

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4 months ago, # |
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I can't understand problem F. Why in this sample case

2
1 9
4 5

The output is

0 0

?

I thought, these scenarios is possible:

  • 1 -> Lee 2 -> Ice Bear 4 -> Lee 8 -> Ice Bear 16 (Lose) -> Next Round 4 -> Ice Bear 5 -> Lee 6 or 10 (Lose)

  • 1 -> Lee 2 -> Ice Bear 4 -> Lee 8 -> Ice Bear 9 -> Lee 18 (Lose) -> Next Round 4 -> Lee 5 -> Ice Bear 6 or 10 (Lose)

Hence Lee can win and can lose too.

Where did I go wrong? Or maybe, what does it mean by "can be the winner independent of Ice Bear's moves or not" and "can be the loser"?

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    4 months ago, # ^ |
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    Here "can be the winner" means that Lee has a strategy where he will definitely win and Ice Bear can't prevent this. Similarly for "can be the loser".

    Indeed, the problem statement is a bit ambiguous.

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4 months ago, # |
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How to solve D for n <= 10^18 without matrix multiplication?

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4 months ago, # |
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https://codeforces.com/contest/1369/submission/84784718 O(N + M) solution for E. It's basically finding an ordering such that if you use that order to direct the edges, out_degree[i] <= a[i]. So you can code it in almost the same way as you would code finding a topological ordering.

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4 months ago, # |
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hello sir, the editorial is awesome and one more thing you provided all code in python too. Thanks a lot sir

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4 months ago, # |
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E question can be solved using data structures

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4 months ago, # |
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Editorial with complete proof is good. It helps me with my poor math......

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4 months ago, # |
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Div2D : Can someone please explain me why are we adding 1 extra claw every 3 levels?

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4 months ago, # |
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Thank you for amazing contest and detailed tutorial!

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4 months ago, # |
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I have a better solution for D(which is close to phrasing the question in another way),

you can think of nodes in 3 — levels

level1 — it is just one node without any children (when it moves to level 2 it gives rise to 1 another node)

level2 — it is node with one child (when it moves to level3 it gives rise to 2 other nodes)

level3 — this node will be our matter of interest which won't give rise to any other node but it signifies atleast one claw(4 nodes)

you can compute level1 ,level2 ,level3 nodes for height-MAX_n using simple dp

level3[i] = (level3[i-1]+level2[i-1])%MOD

level2[i] = level1[i-1]

level1[i] = (level1[i-1] + level2[i-1]*2)%MOD

now precompute all answers for 1 <= i <= 2* 10^6,

which is easy to compute using simple observation, for some i , ans[i] = (ans[i-3] + level3[i] — level3[i-1])%MOD

which means the, if you include claws which changed from level2 to level3 during transition of height i-1 to i , you cannot include nodes which became level3 at height i-1 but you can add nodes which became level3 at height i-2 but not at i-3 ...so on (which is just ans[i-3])

now output ans[height]*4

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4 months ago, # |
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[video editorial for third question] — (https://www.youtube.com/watch?v=5V-RiprjfJQ)

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4 months ago, # |
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In the announcement of the Contest, Author should have written in the name of Lee as its heading.Lol!

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4 months ago, # |
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For question C, can anyone please tell me why this solution is giving wrong answer and why the other one is being accepted. They both are doing the exact same thing.

Wrong Solution — https://codeforces.com/contest/1369/submission/84832565 Right Solution — https://codeforces.com/contest/1369/submission/84832850

Thank You!!

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4 months ago, # |
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Video Solutions for A B C Hope u guys like it :)

Problem A:

Problem B:

Problem C:

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4 months ago, # |
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Nice explanation for solution B.

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4 months ago, # |
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In problem E, I still can't figure out why the situation where si>wi for all i has no solution after I read Complete Solution, could someone explain it in more detail?

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    4 months ago, # ^ |
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    The easiest way to look at it is that in this situation you won't be able to fix the last friend in the permutation. Suppose we fix some friend as the last one in the permutation with food choices as (x,y) , now there will be 0 x left and 0 y left when it is his/her turn because sx-1 times people have eaten X before him and as sx>wx , hence we will have 0 'x' left and same argument for 'y' , so there cannot exist any last friend which implies there is not solution.

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4 months ago, # |
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@DeadlyCritic Apparently the naive n^2 AC's in java for B: 84798072

I guess the moral of the story is make n 3e5 in the future :(

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    4 months ago, # ^ |
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    This is not n^2. It's linear.

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      4 months ago, # ^ |
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      ArrayList.remove() is O(n) and that is called O(n) times, so it is n^2.

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        4 months ago, # ^ |
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        Yes but it depends from which index we are removing. We say it's O(n) because we need to shift the elements after the index of removal. But here we can clearly see that this person is removing either the last element of the list or 2nd last. So in the worst case time taken will be O(2n) not O(n^2). :)

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          4 months ago, # ^ |
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          That's not true. If the string is 100000... then they are removing the second element each time, so it is n^2. You see this by running it locally on a 1 followed by 10^6-1 0s and it will take forever.

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            4 months ago, # ^ |
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            Yes you test case is right. It will give TLE. Sorry!. but this is true best complexity for removal is O(1) when we remove from the last index. You can see this

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              4 months ago, # ^ |
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              Yeah, I know, I'm a java main. My point is that this code is the n^2 and AC's anyway even though it shouldn't because the max n is too small.

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    4 months ago, # ^ |
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    Interesting, thanks for letting me know.

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4 months ago, # |
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worst D in a contest ever. Really stupid.

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Could you please in Problem F, elaborate how did you conclude that

$$$w_{s,e} = w_{s,\frac{e}{4}}$$$
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    4 months ago, # ^ |
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    Because whoever wins that game gets to play in (e / 4, e / 2], thus wins.

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4 months ago, # |
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For problem D, consider the following dp: let a[n] be the answer if we color the root, and b[n] be the answer if we do NOT color the root. They obey the recurrence given in the code below. Then the solution is max(a[n], b[n]). But this does not coincide with the results using the code from the editorial. What am I missing?

int N = 20;
vector<ll> a(N+1); //use root
vector<ll> b(N+1); //do not use root
vector<ll> c(N+1); //editorial
a[2] = 4; b[2] = 0; c[2] = 4;
for(int i=3; i<=N; i++){
    a[i] = 2*b[i-2] + b[i-1] + 4;
    b[i] = 2*a[i-2] + a[i-1];
    c[i] = 2*c[i-2] + c[i-1] + (i%3==2) * 4;
    cout << i << " " << max(a[i], b[i]) << " " << a[i] << " " << b[i] << " " << c[i] << endl;
}
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    4 months ago, # ^ |
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    Because you are thinking $$$unrooted_n = rooted_{n-1} + 2*rooted_{n-2}$$$. This is not correct. $$$unrooted_{n-2}$$$ might be better than $$$rooted_{n-2}$$$. In other words, for $$$unrooted_n$$$, you have the capability to take the children rooted, but it is not mandatory. You are making it mandatory. This wrongful enforcing causes the problem.

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Can anyone explain the reason behind adding (i%3==0) in problem D?
The explanation seemed a bit less explanatory! (and without any pictures)
I almost figured out the solution during the contest except this part :\

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4 months ago, # |
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Can anyone please explain me the approach of problem D? Thanks :)

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    4 months ago, # ^ |
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    If you start constructing the tree, you will find a pattern.
    The tree of level "i" will have a subtree of level "i-1" in the middle and 2 subtrees of level "i-2" on the 2 sides. The structure will be exactly the same as obtained for levels "i-1" and "i-2".
    Thus, dp[i] = dp[i-1] + 2*dp[i-2] + (i%3==0)
    The last part comes from the fact that after every 3 steps, the root claw becomes available to use.
    Credits for the explanation of the last step: anshumankr001

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    4 months ago, # ^ |
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4 months ago, # |
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what's wrong with my solution of C? 84837182

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4 months ago, # |
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Small improvement of tutorial's C code. If you want to sort array in descending order, instead of sort and reverse (also you could write std::reverse), you can use sort(v.rbegin(), v.rend()) for vectors or sort(a, a+n, greater) for arrays

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4 months ago, # |
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I solved problem D by keeping count of nodes with 0,1,3 children. The claws (nodes with 3 children) at level n-3 do not overlap with the bottom most claws at level n and can simply be added to the answer of level n . The bottom most claws will contribute 4 yellow nodes each. If we keep track of how many claws there are at level n we can find our answer.

For every n counts are updated as -

cnt_0[n] = cnt_0[n-1] + 2 * cnt_1[n-1]
cnt_1[n] = cnt_0[n-1]
cnt_3[n] = cnt_1[n-1]

ans[n] = cnt_3[n] * 4 + ans[n-3]

84837417

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4 months ago, # |
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Can someone please explain why my code for C doesnt work?

Link to code -> (https://codeforces.com/contest/1369/submission/84837647)

Explanation -> I first sorted the array "a" in descending order, and "w" in ascending order.

Then i calculated the sum of all elements in a. Now if some w[i]=k,(k>1) then we do not take into account (k-2) elements in the array a. For eg if w[i]=3 then one element is not added to the answer, only the maximum and minimum ones are.

So i calculated how many elements are "lost" this way in the variable lose.

Now to find the optimal answer, the "lose" number of elements must be subtracted from the sum of all elements, starting from the second smallest element (since the smallest element has to be added to the answer at some point since it will be the minimum of any subsequence it falls in).

Also if w[i]=1 then the greatest number shall be added to the sum again.

Any help is appreciated. Thanks!

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4 months ago, # |
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Four methods for Problem D:

Method 0. using dp as the Editorial given: $$$a_i = a_{i-1} + 2 \cdot a_{i-2} + (i \% 3 == 0?4:0)$$$ (use $$$a_i$$$ instead of $$$dp_i$$$ for short) Code: 84808226

there is a typing mistake in Brief Solution of D: $$$(i \% 3 == 0?4:0)$$$ instead of $$$(i \% 3 == 0?1:0)$$$ DeadlyCritic

Expand the above recurrence formula we have

$$$ \begin{pmatrix} a_{3n+2} \\ a_{3n+1} \\ a_{3n} \\ 1 \end{pmatrix} = \begin{pmatrix} 5& 6& 0& 12 \\ 3& 2& 0& 4 \\ 1& 2& 0& 4 \\ 0& 0& 0& 1 \end{pmatrix} \begin{pmatrix} a_{3n-1} \\ a_{3n-2} \\ a_{3n-3} \\ 1 \end{pmatrix} $$$

with $$$a_0 = a_1 = a_2 = 0$$$, and $$$A$$$ indicate the coefficient matrix.

Methods below are $$$O(\log n)$$$ time complex since size of $$$A$$$ is a constant.

Method 1. using fast matrix power we can get $$$a_{3 \lfloor \frac{n}{3} \rfloor + 2}, a_{3 \lfloor \frac{n}{3} \rfloor + 1},a_{3 \lfloor \frac{n}{3} \rfloor}$$$, and $$$a_{3 \lfloor \frac{n}{3} \rfloor + n \% 3}$$$ is the answer

Method 2. It is well known that If you know the characteristic polynomial of matrix, then you can use polynomial multiplication instead of matrix product to get $$$(a_{3n+2},a_{3n+1},a_{3n},1)^T$$$ which is faster that Method 1, especially when the size of $$$A$$$ becomes bigger. Best method in general as I know (can't use NFT, since neither size of A is big nor 1,000,000,007 is NFT-friendly.)

Method 3. Note that the special $$$A$$$ can be diagonalized, thus there exist an invetible matrix $$$X$$$ such that

$$$ X^{-1} AX = diag(8,1,0,-1) $$$

thus we have

$$$ X^{-1} \begin{pmatrix} a_{3n+2} \\ a_{3n+1} \\ a_{3n} \\ 1 \end{pmatrix} = (X^{-1}AX) X^{-1} \begin{pmatrix} a_{3n-1} \\ a_{3n-2} \\ a_{3n-3} \\ 1 \end{pmatrix} = (X^{-1}AX)^n \; X^{-1} \begin{pmatrix} a_{2} \\ a_{1} \\ a_{0} \\ 1 \end{pmatrix} $$$

We can calculate $$$X$$$ above by hand or using following SageMath Code:

A = matrix([[5,6,0,12],[3,2,0,4],[1,2,0,4],[0,0,0,1]])
A.eigenvalues()
X = A.eigenmatrix_right()[1]

Actually we can calculate $$$A^n$$$ with only 3-lines SageMath Code:

n = var('n')
A = matrix([[5,6,0,12],[3,2,0,4],[1,2,0,4],[0,0,0,1]])
A^n

and the last column of $$$A^n$$$ is

$$$ \begin{pmatrix} a_{3n+2} \\ a_{3n+1} \\ a_{3n} \\ 1 \end{pmatrix} = \begin{pmatrix} \frac{32}{21} \cdot 8^{n} - \frac{2}{3} \, \left(-1\right)^{n} - \frac{6}{7} \\ \frac{16}{21} \cdot 8^{n} + \frac{2}{3} \, \left(-1\right)^{n} - \frac{10}{7} \\ \frac{8}{21} \cdot 8^{n} - \frac{2}{3} \, \left(-1\right)^{n} + \frac{2}{7} \\ 1 \end{pmatrix} = \frac{1}{21} \begin{pmatrix} 2^{3n+5} - 14(-1)^{3n+2} \\ 2^{3n+4} - 14(-1)^{3n+1} \\ 2^{3n+3} - 14(-1)^{3n} \\ 0 \end{pmatrix} + \begin{pmatrix} -\frac{6}{7} \\ -\frac{10}{7} \\ \frac{2}{7} \\ 1 \end{pmatrix} $$$

Code: 84843466

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    4 months ago, # ^ |
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    By using another approach based on the ideas in this blog, thinking of the +4 as impulses in the recurrence f(x+2) = f(x+1) + 2*f(x) we can end in a nicer formula using geometric sums: $$$\frac{4(\frac {1 - 8^{floor(N/3)}} {1-8} \cdot 2^{N mod 3 + 1} - \frac {1 - (-1)^{floor(N/3)}} {1-(-1)} \cdot (-1)^{N mod 3 + 1})}{3}$$$

    Code

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      4 months ago, # ^ |
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      That's the intended solution, nice!

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      4 months ago, # ^ |
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      Thanks! I understand how you get the geometric sums, by calculating generating function. Really much nicer!

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      4 months ago, # ^ |