Livace's blog

By Livace, history, 2 years ago, translation, In English,

Hi, Codeforces!

On monday, October 23rd, at 18:35 MSK rated Div. 2 round will be held. Participants from first division can take part out of competition.

The problems for this round were prepared by me. Thanks to Daniil (qoo2p5) Nikolenko, Nikita (neckbosov) Bosov, Alexander (Alladdin) Proskurin, AmirReza (Arpa) PoorAkhavan, Ildar (gainullin.ildar) Gainullin, Alexey (ashmelev) Shmelev for help in preparations and testing problems, Ann Izyumova for help with translation, Nikolay (KAN) Kalinin for the round coordination and Mike (MikeMirzayanov) Mirzayanov for amazing platforms Codeforces and Polygon.

You will have 2 hours for 6 problems.

Hope you will enjoy problems! Good luck!

Upd1: Editorial

Upd2: Congratulation to winners!

Div1

  1. Farhod_Farmon
  2. Gritukan
  3. KrK
  4. rajat1603
  5. chemthan
  6. I_love_Maria_Ivanova

Div2

  1. I_want_au
  2. GTMD_UESTCACM
  3. SORONGON
  4. Sheng_D_Bao
  5. wxh010910
 
 
 
 
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2 years ago, # |
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Spoiler
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Is it rated?

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2 years ago, # |
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As far as I remember, all the Codeforce Rounds in the past eight months have been rated unless otherwise stated before, during, or after the Contest.

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    2 years ago, # ^ |
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    "stated before, during, or after the Contest."
    lmao

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      During or after the Contest due to sudden troubles during the Contest such as significant error(s) in some problems or temporary server failure.

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If people think it is cool to comment "is it Rated ?" in here. You guys are wrong -__-

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seriously guys , how two comments with same question and in the same time and they have the same color :/ , the first got upvotes and the other one got downvotes ?

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    2 years ago, # ^ |
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    A simple plausible explanation is that what is disliked about the second one is not its content, but its repetition, i.e. the second one is a redundant comment.

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    2 years ago, # ^ |
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    probably the uppercase :-P

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    2 years ago, # ^ |
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    Fake accounts :p

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    2 years ago, # ^ |
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    Probably because the all caps comment looks sarcastic.

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    2 years ago, # ^ |
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    Possible reason may be, The one with more Down votes is 'Grey Coder' and 'Blue' on other side(colour matters).

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** ** *****?

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2 years ago, # |
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Is it Rated Contest or unrated???

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To all people who are asking whether the contest is rated: check the tags.

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del

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why the registration process for the contest is off.

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The increase of the legendary word.

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how to trigger everyone in 3 words: is it rated?

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    2 years ago, # ^ |
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    43 dislikes these people are actually autistic haha

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Hope problem statements are understandable easily and the contest will enjoyable.

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I'm one point below div 1. So to me seems like a notorious coincidence

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    2 years ago, # ^ |
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    I hope you don't lose points in this contest otherwise it won't be a NOTORIOUS coincidence.

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    2 years ago, # ^ |
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    Had the same situation few rounds ago, but was 5 points far from div1 :D

    The rest of the story isn't really cool xD

    Wish you good luck ^^

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      2 years ago, # ^ |
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      that was very comforting for him/her i guess. lol XD !!!

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        2 years ago, # ^ |
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        Well, he became candidate so I think that was very comforting xD

        Congratulations ^^

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I hope the difficulty of problems decreases gradually. In the last few contests, I have noticed a massive decrease in the number of submissions for the last two problems. The image shows one such contest. Note the actual number of submissions during the contest were quite less than that in the image. The same thing happened in yesterday's cook off on codechef.

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Oh, God why?

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    2 years ago, # ^ |
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    I hoped nobody would notice that. I think it's the most stupid mistake somebody made in announcement.

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    2 years ago, # ^ |
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    Stupid community. Even CIS chat is better.

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      2 years ago, # ^ |
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      You are yourself part of this community.

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      22 months ago, # ^ |
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      CIS? What does the acronym stands for? My wildest guess is

      Spoiler
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Scoring distribution?

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    2 years ago, # ^ |
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    Like always, scores will be disclosed only after start of contest. :(

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delayed :P

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Delays, classic

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Wait for 10 minutes more :(

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Looks like they wanted the total registrations to reach 7k :P

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this delay means a lot of inqueue :(

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Thanks for the delay. I've just noticed that I didn't register for the contest before the delay.

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Please, no more delays.

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problem B. boring statement with less sample test. annoying.

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How to solve Div2 B Nikita And Strings.i didnt' able to solve it plzz help

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    2 years ago, # ^ |
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    my solution is to write two nested loops with the end of the first part and the begin of the third part and maximize your solution and you can calculate the length of the final string with pre-calculations in O(1)

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    brute force for 2 points to split the string into 3 parts..

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      That gives TLE? I did same thing and got TLE

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        yes, it will give TLE if you use any further loop inside 2 nested loops to iterate the string. you can use the 2nd loop to count the frequency of 'a' and 'b' at 2nd and 3rd part of the string. see my code.

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          I saw your code after I commented and realized my mistake. Thanks :)

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Can anyone explain the problem 3? It seem simple but got WA.

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    it didnt' seem simple to me

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      I thought that we simply bomb n to 1, and then bomb 2 place.

      That would destroy all tanks.

      But got WA

      Can anyone exaplain why this is wrong?

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        2 years ago, # ^ |
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        For example: if you bomb the 7th, tanks can move to 8, and they'll survive

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        2 years ago, # ^ |
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        When you bomb N, and then N-1, some tanks from N-1 might move to N after bombing

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          2 years ago, # ^ |
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          I thought according to the problem, tanks that are not in pos. 1 must move to n-1. So tanks in n-1 must all move to n-2.

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          2 years ago, # ^ |
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          bt it is given that (a tank in the cell n can only move to the cell n - 1, a tank in the cell 1 can only move to the cell 2)

          EDIT : oh...that was only for both corners..

          damn :(

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          2 years ago, # ^ |
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          "a tank in the cell n can only move to the cell n - 1, a tank in the cell 1 can only move to the cell 2"

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    2 years ago, # ^ |
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    Problem C explanation :
    First drop bombs on even numbered cells, then on odd numbered cells, then again on even numbered cells(1-indexing). My solution 31644671

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    2 years ago, # ^ |
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    Put bombs first on all even number positions, then odd number positions and finally even number positions. In this way we can make sure every tank is put bombs exactly twice.

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      2 years ago, # ^ |
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      How did you prove that it is optimal?

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        Well, how i solved this question was as follows : I first of all ran a dp solution on my computer for 1<=n<=200( as the dp solution in O(n*n) ), and found out that it is always optimal to pick 2nd number from the start, and therefore I ran a recursive solution. Here is the link to my solution: http://codeforces.com/contest/877/submission/31652910

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        2 years ago, # ^ |
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        Well, you can assume that each tank is in either Even or Odd cell. If its blasted, it goes from Even->Odd or Odd->Even

        So, just had to print minimum of Even->Odd->Even or Odd->Even->Odd, of which for odd n, first is smaller

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        2 years ago, # ^ |
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        Every tank has to be dropped a bomb at least twice. Using this method every tank will be dropped a bomb exactly twice. So no bomb is wasted(i.e. No tank is dropped a bomb more than twice). This has to be optimal.

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How to solve B ??

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    Iterate for ending position of portion1 and starting position of portion2. Use prefix sum to speed up.

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was O(N*M*2*log(n*m)*8) too much for D

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How to solve C ?

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    I had the following logic : - Bomb all even cells. Now all tanks are on odd cells - Bomb all odd cells. All (intial) even cells are destroyed. All odd cells are on even cells - Bomb all even cells.

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      The last logical step is to show that there is no better strategy.

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        How do you prove that there is no better strategy?

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          I don't know. I just know that the lower bound is n (we must hit every cell) and upper bound is n + n / 2. The truth is somewhere in-between :)

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How to solve D?

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    I did bfs after putting all empty cells in set rowwise and column wise and deleting cells from set after pushing in bfs queue but got TLE on test 6

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Does this problem have a logN per query solution? :

ask for number of occurences of x in range l,r

update range l,r by some d

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A was clearly the hardest problem, as I didn't really understand the following sentence. :(

"It is known, that problem is from this contest iff its name contains one of Alex's friends' name exactly once."

So I made two submissions, both of which are incorrect but passed pretests. :P

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    then what about C,d and E did u solve it

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    it didn't clearly i couldn't understand what once his friend or name of friend

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    Got hacked on A. For the first time I was actually thankful that I was hacked because it made me realize my code is faulty and reread the question.

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Why in D grid is 1000 x 1000 (when time limit is 2 sec), it is too big for slower languages. Why not 500 x 500?

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    nmk solutions can pass then

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      If k is big enough then it wouldn't pass 500 x 500? Look there are no solutions with Python nor PyPy, and my Perl solution O(nm) was too slow too :(

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E was just a tree version of http://www.spoj.com/problems/LITE/

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Anyone can tell me how they did C? I did a recurrence but I got WA. if its odd then hit the middle one, then recursively destroy (i, mid-1) and (mid+1, j). If even then if mid=floor(n/2) destroy (i,mid) then destroy (mid+1, j) then hit mid. Anyone sees why this is wrong or what the correct answer is?

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    I destroyed all even first. Then all odd. Then all even again. Passed Pretests. I dont know about sys tests.

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    it isn't really a recursive structure since after hitting mid, you leave a guy with 1 health on mid-1 or mid+1, but all the other guys have health 2

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    2 years ago, # ^ |
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    1. Hit the even indices
    2. Hit the odd indices
    3. Hit the even indices

    Then you'll get a plan with n + n/2 steps

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    I had same mistake. I used almost the same solution.

    This solution definitely constructive a valid answer, but not in minimum number of bombs needed. I believe it fails when there is a configuration where some element appears once. Just check answer when N = 10.

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Is the intented solution for F O(NsqrtN), and if it is, why are solutions with that complexity getting a TLE?

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hack for A-> DDanilanil :)

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    2 years ago, # ^ |
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    Some other people forgot to check if the name does not appear more than once in the problem name. Simple hack : AnnAnn but your case is more tricky like AAnnnn since some people removes the first occurrence of the name and then recheck if it is in there one more time.

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How to solve F?

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I do really think it's much easier

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Any corner case for Div2 B ?

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Can't D be done in O(n*m)? I see a lot of solutions with complexity O(n*m*k) passing pretests.

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    Carefully observe the 'break' statement and calculate the complexity.

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      No, I noticed pure O(n*m*k) solutions without any break statements passing the pretests.

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        I have passed the pretests without any break (see 31647369). However, I got AC after using breaks (31658960), but I do not think I should have gotten AC on a solution of O(nmk).

        I think that my solution would TLE on a test of the form below. The answer is -1.

        1000 1000 1000
        ... (..) ...
            (..)
        ... (..) ..#
        ... (..) .#.
        1 1 1000 1000
        
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          2 years ago, # ^ |
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          I got TL on such test and I really don't understand why your solution got AC. That's mine — 31650304. It seems to me that they are equal to each other.

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            2 years ago, # ^ |
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            Your source code has the break (actually return 0) when you pop (x2, y2) from the queue. My source breaks a little earlier, when (x2, y2) is pushed into the queue.

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              2 years ago, # ^ |
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              Yeah! Thanks a lot! Actually, I have noticed it a minute ago too. Then, the question why your solution works faster then mine? Difference ~ 500 ms. Because of using vectors? 31661871

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What should be the output for this in problem A : "DanilDanilOlya"

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    NO

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      but the question said, it will be "YES" if ANY ONE OF THE FRIENDS come exactly once.. Little confused, if exactly one of the friends should come exactly once, or any one of them should come exactly once !?!?

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        2 years ago, # ^ |
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        The last sample helps to understand which of them is true, doesn't it?

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          In last two of the friends got exactly once..so it should be NO. But what if any one friend came twice but another came once... (found it ambiguous).. < Although System Test Failed on 35 >

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How to Solve C ?

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Ended up only solving D. A failed at systest because of stupid sizetype being unsigned(so 1-2=2^32-1 which isn't <1), B failed because i stopped searching for more As/Bs after finding B/A, while D was easy BFS(at least for me). I don't understand how did such a small amount of people solve D.

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    hi there, can you please tell me why this solution gives MLE? how to get rescue from this?

    31687090

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Codeforces HACK round #442 (Div. 2)

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If someone would be kind enough to tell me what went wrong with my E submission ( http://codeforces.com/contest/877/submission/31648193 ). Thanks

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Is there something wrong with codeforces's servers ???

I have submitted the same code 3 times and each time I got run time error even though I didn't change anything in the code.

Can someone tell me why 31659189 solution is giving runtime error ?

My idea is to turn the tree to an array and then turn this array into sqrt(n) Buckets and then preform the queries on them to get a n * sqrt(n) solution

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can anybody plz tell me how to solve div2 E.

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Can anyone explain me why? 31638541

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oh boy!

I got WA on D test 31

I used a simple BFS too and can't find any bugs in my code or algorithm

could sb help me with it :'(

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    2 years ago, # ^ |
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    same here

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    You have to maintain visited array for all the 4 sides, mine code also failed due to this but now it passed by maintaining the above information :(

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      2 years ago, # ^ |
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      Do you have any case to prove this?, I haven't been able to come up with a case that makes fail my submission. And test 31 is too large :'(

      EDIT: I found the case.

      7 14 14
      XXXXXXXX..XXXX
      XXXXXXX...XXXX
      XXXXXX..X.XXXX
      XXXXX.....XXXX
      XXXX..X...XXXX
      XXXXXXXXX.XXXX
      XXXXXXXXX.XXXX
      1 9 7 10
      

      the visit order may be the cause. So you need to memorize the direction too, seen[f][c][d] and that should fix it.

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        I think test cases are little weak as my this solution passes.

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      Well, used 4 ifs instead :P

      dirtyProgramming !

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Can somebody tell why this code gave WA for case 49 — http://codeforces.com/contest/877/submission/31647784 in problem D?

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2 years ago, # |
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Suppose if I have a map of size 4 in Div2 C: and I go with the sequence : 2 1 4 3 2 What's wrong with this? Aren't all the tanks damaged after following this sequence? Please Help!

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    2 years ago, # ^ |
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    The tank on 3 can go to 4 so you would need to bomb 4 again.

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      2 years ago, # ^ |
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      I am sorry, if I am wrong, but we are told that a tank in cell n can only go to n-1 cell, so how would it go to 4, if you could explain this a bit? Thanks!

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        2 years ago, # ^ |
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        It is given in the question that it can go to n+1 or n-1. Only the tank at the border cannot do that.

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    2 years ago, # ^ |
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    forth cell isn't destroyed

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2 years ago, # |
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could somebody plz tell me why this simple bfs code fails on test 31 http://ide.geeksforgeeks.org/5QUsiv Thanks.

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    2 years ago, # ^ |
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    It failed due to the break part in the 60th line. Supose you are in position xi, yi and you are going downwards. Now you are checking (xi+i,y) but this was already visited. Just because you have previously visited (xi+i,y) you cannot assume that the next cell (xi+i+1,y) (or any of the next reachable cells) is also visited, hence the break is incorrect. And without the break you get TLE.

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      2 years ago, # ^ |
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      ok ok. got it. so what i think is if a cell is already visited, better not push it, instead of breaking at the point

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        2 years ago, # ^ |
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        Also, Olya can walk UP to k units, and does not necessarily have to walk k units/hits the end of the room/hits an obstacle.

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2 years ago, # |
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Livace do you hate me? I'm in top 5 div1 :(

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    2 years ago, # ^ |
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    Oh, sorry. I'll fix it.

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    23 months ago, # ^ |
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    you are nothing.... min_25 is best in world after that tourist then peter then W4yneb0t and then moejy0viiiiiv ... you are just a crazy boy ;)

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2 years ago, # |
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I solved B by the DP method. I tried to find the largest "increasing" subsequence. Wrong answer 15. Can anybody prove why my decision is wrong? My solve: 31644137

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2 years ago, # |
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The problems were cool, but the pretests were really annoying since they actually didn't contain anything important (like tests 48 & 49 for D).

Only 2 D's from my room passed system testing. Out of 14!

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    2 years ago, # ^ |
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    Hey! What were those cases ? My solution is failing on Case 30.

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      2 years ago, # ^ |
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      So have you solved the problem? I am always failing on the test 31. What a pity...

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2 years ago, # |
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In the Problem statement for C it was given that If we drop a bomb on the tank in nth cell then the tank will be half damaged and will move to (n-1)th cell.So if we start dropping bombs from nth cell and and go on until first and then in the very last we drop bomb on 2nd then it gives answer (n+1).

for example say we have n=3. So at first we have a configuration 1 2 3. After dropping the first bomb on third cell--> 1 23 _ (3rd tank moved to 2nd cell and half damaged) After dropping the 2nd bomb on 2nd cell --> 12 _ _ (3rd tank is completely destroyed and 2nd is half damaged and moved 1st cell) After dropping the 3rd bomb on 1st cell --> _ 1 _ (2nd tank is completely destroyed and 1st tank is moved to 2nd cell)

This is true for any number of cell.

Have I misunderstood something? I can't also see that problem now as the problem page is showing a error saying "Oops! Something broke on Codeforces. Do not panic. You can try to reload the page or go back to the main page . We already read the megabytes of logs, solving the problem."

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    2 years ago, # ^ |
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    Tank at nth position can move either to the left or right, but only tanks at the border can move to cell N — 1, in case of position N (last) or 2 in case of postion 1

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2 years ago, # |
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After I saw that contest will be only 2 hours long, when 6 problems, I thought maybe contest should be longer. Now I think that contest surely should be longer! Because three problems A, B, D are really tricky so interesting to hack.

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2 years ago, # |
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I feel like I've seen problem E before...

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2 years ago, # |
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I have a question about div2 D.

Here is the statement of concern:

Problem Statement

Does this mean she can run UP to k or she has to run k unless she hits the edge/an obstacle? I assumed it meant she ran k, but my solution — 31656460 failed test case 5.

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2 years ago, # |
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How should i understand div2 B correctly my friends? I thought a beautiful string is a string that can be cut into exactly 3 substrings described as in the problem. What did i do wrong? I can't even understand english nowadays!

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    2 years ago, # ^ |
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    Substrings can be empty.

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    2 years ago, # ^ |
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    I'm fought up at test 3 bbabbbaabbbb with my result is 6, while the correct answer is 9. How can i produce a substring with 3 (may-empty) parts that part 1 and 3 contain only 'a' and part 2 contains only 'b'? Really in this case i just can see the result are abbbaa or aabbbb and nothing left!

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2 years ago, # |
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in problem B can anyone please explain where my approach is wrong.

I first created blocks of a's and b's with their respective counts and then i consider the current block is the middle element and if it is 'b' then left and right part could be anything (i.e. bba, bbb, aba, abb) and if current block is 'a' then (aaa, aab, baa) and find the maximum among these (to do this i just calculate the total 'a' and 'b' in the left and right substring respectively).

code : http://codeforces.com/contest/877/submission/31666767

please help me where i am wrong !

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    2 years ago, # ^ |
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    If the string form is bbb or abb why not considering adding somemore 'a' blocks after it? You may try the input abbbabbba.

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2 years ago, # |
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Problem F Why unordered_map is even slower than map? Both TLE on test 11

unordered_map use 1450ms on test 10 map use only 576ms array use 249ms on test 10

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    6 months ago, # ^ |
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    What's the data size? In small data set, map is faster than unordered_map indeed.

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2 years ago, # |
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31659292 877B - Nikita and string Hey can somebody help me find out what is wrong with my solution to B — Nikita and String. I got wrong answer in test case 37. Thanks.

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    2 years ago, # ^ |
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    It's easy to see. You did not consider the case when the string contains only a's.

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2 years ago, # |
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In div2 D, what ever i do, i always get MLE at test case 5 with this solution:31687090 .why? How can i fix it?

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    2 years ago, # ^ |
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    Make sure you didn't enter in an infinite loop with your bfs.

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2 years ago, # |
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Where Can I find Tutorials ?

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2 years ago, # |
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anyone can help to explain why in second sample case for problem C. Slava and tanks can destroy all the tanks? The solution doesn't drop a bomb in 4th cell.

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    2 years ago, # ^ |
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    There are only 3 cells

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      2 years ago, # ^ |
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      Oh my bad, I mistakenly saw 4 as an input instead of output. Thanks

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2 years ago, # |
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