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Rudro25's blog

By Rudro25, 3 years ago, In English

Contest link : https://codeforces.com/gym/102942

Announcement link : https://codeforces.com/blog/entry/87209

Finally, The round is over smoothly :) We hope everyone had fun and enjoyed the contest!

A. Directional Move

Tutorial
Solution
Tester Solution

B. Make All Odd

Tutorial
Solution

C. Team

Tutorial
Solution
Tester Solution

D. XOR Game

Tutorial
Solution
Tester Solution

E. Password

Tutorial
Solution
Tester Solution
Tester Solution 2

F. Offer

Tutorial
Solution
Tester Solution

-If have any more query, then please comment.

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3 years ago, # |
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thanks for this contest and hope for more contest like this.

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3 years ago, # |
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Great round. For D, if x and y have no common bits then answer is NO else YES. So this will also work

if(x & y) cout << "Yes";
else cout << "No";
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    3 years ago, # ^ |
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    this will also work!

    cout << ((a | b) == (a + b) ? "No" : "Yes") << "\n";
    
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    3 years ago, # ^ |
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    thank you for this . I came up with the same logic , but didn't know how to proceed with bit manipulations.

    So used the basic method to calculate every bit by n%2 and check whether both of them are 1 or not

    if(a%2 && b&2)
    {
    cout<<"Yes"
    return;
    else
    a=>>1;
    b=>>1;
    }
    
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3 years ago, # |
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Thanks for the round and quick editorial

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3 years ago, # |
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Thanks for the round waiting for the next one

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3 years ago, # |
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Hoping to solve upto D in div2 contest also some day. btw thanks for this contest and hope for more contest like this.

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3 years ago, # |
  Vote: I like it +15 Vote: I do not like it

Nicely done, thanks for the contest!

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

E was really a good problem for learning dp, thanks for the round.

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3 years ago, # |
  Vote: I like it +3 Vote: I do not like it

We can also solve E with combinatorics. Combination with repetition to be exact. Suppose the string is like, 1----5. We can place any digit from this, {1,2,3,4,5} set in increasing order in 4 place between 1 and 5.

Let, n = number of place to fill. m = size of the set. Possible ways will be (n+m-1) Choose n.

For practice: https://codeforces.com/contest/1288/problem/C

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    3 years ago, # ^ |
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    I thought on similar lines but wasn't able to get my code working as intended. I'd appreciate it if anyone out here could look into the following submission.

    Link: https://ideone.com/U8cjmP

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      3 years ago, # ^ |
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      Try checking the validity of the result explicitly.

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      3 years ago, # ^ |
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      The problem is with the size of your fac array which is 100005 but according to the problem n<=10^6 which makes n+k-1<=100009(and it is less than 100005).

      I changed 100005 to 100020 and it got Accepted

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    3 years ago, # ^ |
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    Thanks for the formula, I got stuck at this and couldn't find this formula. Can you please provide me an explanation or a link where this formula can be find? Thanks for the given problem too.

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      3 years ago, # ^ |
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      Stars and bars

      Imagine you need to fill a sequence of integers of size $$$10$$$ using only $$$1$$$, $$$2$$$, or $$$3$$$ so that the sequence is non-decreasing. As they are non-decreasing, all $$$1$$$'s have to appear in front of all $$$2$$$'s and so on, so we don't need to care about the order of these integers and we need only the number of occurences of each $$$1$$$, $$$2$$$, and $$$3$$$. Thus, we need to find number of solution of: $$$count_1 + count_2 + count_3 = 10$$$, with $$$count_0, count_1, count_2 \geq 0$$$.

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      3 years ago, # ^ |
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      Check this out: https://www.youtube.com/watch?v=4_je4mXUCGA Hope it will be helpful. I learned it from Coursera. Someone uploaded the video in YouTube too.

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      3 years ago, # ^ |
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      Consider this example:

      1 — — — 6

      In this we have to fill 3 places and to fill those 3 places we have 6 elements. Now observe that we only need to choose the elements with which we want to fill the gaps. We don't need to decide the order as that is fixed.

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    3 years ago, # ^ |
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    I also tried to solve it with combinatorics. 105596767 This is the code it's failing test case 4. So can you help? if you find something. Thanks in advance.

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3 years ago, # |
Rev. 2   Vote: I like it +6 Vote: I do not like it

Thanks for the contest, had fun doing it!

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3 years ago, # |
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Please make the submissions public, or atleast the testcases. It helps a lot in debugging silly mistakes.

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    3 years ago, # ^ |
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    I already told this secondthread sir. May be he will open soon , if possible sir.

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3 years ago, # |
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Could anyone explain F in a more detailed way? It would be much appreciated.

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    3 years ago, # ^ |
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    It's just 2 pointer approach, we can keep on increasing our right pointer while maintaining frequency and cost, until the cost becomes greater than k. At this point we can start incrementing our left pointer and decreasing the cost. The number of elements will right-left+1. Now we keep on repeating the prcoess.

    Code
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3 years ago, # |
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If in problem D, we were asked to maximize the score of Bob, how will we solve it ?

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    3 years ago, # ^ |
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    I think You can iterate from the highest to lowest bit, and try to set it, if possible.

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3 years ago, # |
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The contest was great! Although I had come up with a similar idea as editorial, I had a hard time proving D during contest. I still can't wrap around why case 2 always is true. Could anyone please explain?

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3 years ago, # |
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Nice Contest..problem set was really good.

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3 years ago, # |
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Very nice contest! Want more! Especially thanks for the good E task.

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3 years ago, # |
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The F can also be solved using the bit + binary lifting technique.

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3 years ago, # |
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can you please add more test cases? bacause it is just checking with one test case and getting accepted?

so now i don't even know whether my solution is correct or not?

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    3 years ago, # ^ |
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    You have the permission to see only 1 test case, sir. May be it's the 'gym' contest rule.

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3 years ago, # |
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For C, why is the following approach incorrect? Answer is at least equal to elements >=k. Now put other elements in a list and sort it. Now iterate over every element, for every a[i], use lower_bound() on k-a[i] in range [i, list.size()] to find if such element exists. If it does, mark it used and increment answer. Why does this give a WA?

Code
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    3 years ago, # ^ |
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    can you please provide the code by spoiler! no permission to see other submission :(

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      3 years ago, # ^ |
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      Sure! My bad

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        3 years ago, # ^ |
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        Failed test case: 1 10 5 1 3 2 1 4 3 4 2 3 1

        Expected: 4 but found 6.

        May be the problem is — When you find a pos by lower_bound it can be also used before but you not checked that. may be you can use set and remove every used element then you can use lower_bound easily.

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          3 years ago, # ^ |
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          Ah right, I think the problem exactly is what you pointed out, I should definitely be removing those used values from my list. Thanks! Orz

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3 years ago, # |
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Let's consider a variation of problem F. Now in the subsegment [l,r] instead of making the cost of all repeating elements in that subsegment zero if we were allowed to take only one of the repeating elements in that subsegment and make its cost equal to zero. Rest all the conditions remain same as mentioned in the problem F. Then can someone please provide an insight on how we could have solved it.

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3 years ago, # |
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Hello , Thanks for this Contest . Can you Please Update the Settings so that we can we view all the test case , My Code for F gave wrong answer on test case 3 but I cannot access that test case . Thanks .

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    3 years ago, # ^ |
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    secondthread sir told that's may be impossible sir. but you can provide your code, i can check that by using polygon which on failed (:

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3 years ago, # |
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We need more contests like this. Contests like today's destroy us and you pick us up. Thanks for organizing this man!

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3 years ago, # |
Rev. 3   Vote: I like it +1 Vote: I do not like it

So, my understanding of the solution of $$$D$$$ is as follows:

If $$$a$$$ and $$$b$$$ are given such that there are no common bits between them, the answer is $$$NO$$$, because there are no 2 integers $$$1 \leq c \leq a,$$$ $$$1 \leq d \leq b$$$ s.t. their xor will be strictly greater than the xor between $$$a$$$ and $$$b$$$.

If $$$a$$$ and $$$b$$$ are given such that there are common bits between them, the answer is $$$YES$$$, because we can always find 2 integers $$$1 \leq c \leq a,$$$ $$$1 \leq d \leq b$$$ s.t. their xor will be strictly greater than the xor between $$$a$$$ and $$$b$$$.

But I don't understand why this should always be true (i.e. for 2 integers $$$a$$$ and $$$b$$$ s.t. there are common bits between them, why is it that we can always find 2 integers $$$1 \leq c \leq a,$$$ $$$1 \leq d \leq b$$$ which don't have common bits between them, s.t. their xor will always be strictly greater than the xor of $$$a$$$ and $$$b$$$), can anyone please provide a more detailed explanation of this or a proof?

Edit: deleted some erroneous stuff I thought was correct..; also, thanks a lot @doubleux, that last paragraph (especially) really made it very clear and obvious that the "strategy" for the solution of $$$D$$$ would always work.

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    3 years ago, # ^ |
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    Notice, that the maximum Xor between any two numbers is always less than their sum, the maximum arising when no two bits are common.

    Thus, if no bits are common, Xor of a and b is a+b, which is maximum that you can get using numbers less than them. Therefore no pair possible in this case.

    If the do have common bits, a Xor b will be 0 at that position. Now choose c same as a, while for d, change the bit at common position as 0. This will make that bit 1 in their bitwise Xor, thus making it larger.

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3 years ago, # |
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I used the same logic and approach in F but got TLE, why Python why :'-)

Problem F submission

Great tasks tho!

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3 years ago, # |
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I used the chorus property. a^b=c -> a^c=b so (c^d = (a^b)+somenum) -> (c^((a^b)+somenum)=d) And did 1<=somenum<=10. But why +10 was enough I don't understand. can open my eyes?

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3 years ago, # |
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IGNORE

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3 years ago, # |
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Video tutorial for problem D: https://youtu.be/y2ERMJFsyjg

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3 years ago, # |
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please conduct more these type of contest . This is very helpful for beginners.

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5 weeks ago, # |
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Thanks for the contest!