fcspartakm's blog

Codeforces Round #452 (Div. 2)
By fcspartakm, 6 years ago, translation, In English

Hello again, Codeforces!

I'd like to invite you to Codeforces Round #452 (Div. 2). It'll be held on Sunday, December 17 at 09:35 MSK and as usual Div. 1 participants can join out of competition. Note that round starts in the unusual time!

The round is rated.

This round is held on the tasks of the second day of the municipal stage All-Russian Olympiad of Informatics 2017/2018 year in city Saratov. They were prepared by Olympiad center of programmers of Saratov SU. A convincing request to the participants of the municipal stage in Saratov to do not participate in this contest.

Great thanks to Nikolay Kalinin (KAN) and Grigory Reznikov (vintage_Vlad_Makeev) for helping me preparing the contest, to Mike Mirzayanov (MikeMirzayanov) for the great Codeforces and Polygon platform and to Alexey Ripinen (Perforator) for writing solutions.

You will be given six problems and two hours to solve them. The scoring distribution will be published soon. Good luck everyone!

UPD The scoring distribution 500-1000-1500-1750-2250-2500

UPD: The system testing is starting now, but upsolving, virtual participation and viewing solutions and tests will be disabled till the end of the olympiad in Saratov (around 2-3 hours from now). Hope for your understanding. Editorial will be posted after the olympiad as well.

UPD Congratulations to the winners!

  1. Spyt

  2. taew

  3. Fop_zzZ

  4. kabuszki

  5. Join_VNOI_Discord

UPD Editorial

  • Vote: I like it
  • +159
  • Vote: I do not like it

Comments Comments (144)
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6 years ago, # |
← Rev. 2   Vote: I like it +6 Vote: I do not like it

You wrote two consecutive rounds! Wow!

UPD Never mind, I didn't know that the rounds were based on Olympiad. Sorry.

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6 years ago, # |
  Vote: I like it +5 Vote: I do not like it

This round is the based on problems from the same contest as today's contest. Does this mean the round, Div 2 in particular, will be of similar difficulty to today's round? It was a great round (personally anyway), don't get me wrong, but it was definitely easier than normal.

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    6 years ago, # ^ |
      Vote: I like it -11 Vote: I do not like it

    Many people solved problem D and E in today's round.

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    6 years ago, # ^ |
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    LOL dude i thought i was going to end up like you at the end of the contest but i messed up really bad today

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6 years ago, # |
  Vote: I like it -11 Vote: I do not like it

Why is it so late?

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6 years ago, # |
  Vote: I like it 0 Vote: I do not like it

In this month will be some codeforces round

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6 years ago, # |
← Rev. 2   Vote: I like it 0 Vote: I do not like it

1:35 PM in Hanoi.

It would be good if my class wouldn’t start at 3:00 PM... :<

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6 years ago, # |
  Vote: I like it +7 Vote: I do not like it

i hope that this round's problems will be as difficult as the previous round's problems

so as the queue

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6 years ago, # |
  Vote: I like it -8 Vote: I do not like it

First contest without Russians?

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6 years ago, # |
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An UTC+8-round :)

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    6 years ago, # ^ |
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    While is the upd point for each problem?

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6 years ago, # |
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1:35 AM in Cuba , nice!!!

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6 years ago, # |
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I really hope this round will be better than the round before.

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6 years ago, # |
  Vote: I like it +27 Vote: I do not like it

Six problems? Sounds good. And the time is quite friendly to Chinese participants :)

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6 years ago, # |
  Vote: I like it -28 Vote: I do not like it

The classic: Is it rated?

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6 years ago, # |
← Rev. 2   Vote: I like it -15 Vote: I do not like it

All right... It seems that today's problems are much better than yesterday's. I like this round very much. -- Though my B was hacked.

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6 years ago, # |
← Rev. 2   Vote: I like it 0 Vote: I do not like it

6.35am(UK) is so early XD

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

    the last contest in Cuba was at 6:30AM and this contest is at 1:35AM , very early XD

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6 years ago, # |
  Vote: I like it -17 Vote: I do not like it

apparently the link described in the rules,

http://codeforces.com/blog/entry/456 and http://codeforces.com/blog/entry/4088

was not found on Codeforces's server on my end. anyone know how to access this?

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    6 years ago, # ^ |
      Vote: I like it -17 Vote: I do not like it

    ...damn it. it was two links. i'm stupid.

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6 years ago, # |
  Vote: I like it -23 Vote: I do not like it

wh2001_ZY will solve all problems in the contest!He's the best!

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

whan I want to hack, I couldn't see other's solution .

why????

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

    You lock your solutions in the dashboard before hacking other solutions, but beware, after locking your submission you won't be able resubmit any solution after

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

How to solve D and E?

I got no idea for D.

For E, though I was not able to submit because of too much code writing but my idea involved range min query and lazy propagation.

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    6 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    I think E requires some clever use of data structure like set . Couldn't get AC though because of time but I think my approach is correct.

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    6 years ago, # ^ |
      Vote: I like it +2 Vote: I do not like it

    I solved E simulating the array with set and map

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      6 years ago, # ^ |
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      Yes. That's what I did . but got wrong answer in pretest 7 . Finally when I corrected my solution , Contest was already over. screwed up badly !

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        6 years ago, # ^ |
          Vote: I like it +1 Vote: I do not like it

        Sorry man. But dont worry, I also had a bad contest :)

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      6 years ago, # ^ |
        Vote: I like it +1 Vote: I do not like it

      Hi , TwiceBig I was not able to solve E in contest timings Will you share your thinking and solution , that would be really great.. I am attempting the problem now, your method will surely help..

      Thankyou.

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        6 years ago, # ^ |
        ← Rev. 5   Vote: I like it +7 Vote: I do not like it

        Hi. Well basically what I do is simulate the array using a set. For that I store a pair of integers. First frequency and second what position of the array it is. Since set has data stored by minimum, I store The first value as negative. Also I have an extra map for helping me in the merge. While there is something in the array ( the set ) I erase the first value of the set, cause it is the next range that has to be erased. When I remove an element I search the left and right element with lowerbound in the map. And if left and right has same value I merge them. So I have to remove from the set the left and the Right. And add the new pair I just merged. And update the map. And that's all. Hope it helps

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    6 years ago, # ^ |
      Vote: I like it +1 Vote: I do not like it

    33347025, got AC with Disjoint Set Union. Each connectivity component — is segment with equal elements. I kept this components in set, selecting component with maximal size. Time complexity approx O(N log N)

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6 years ago, # |
  Vote: I like it 0 Vote: I do not like it

I thought I finally found the bug right before the end, but still WA on pretest 4! What's pretest 4 on problem D?

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    6 years ago, # ^ |
    ← Rev. 2   Vote: I like it 0 Vote: I do not like it

    n is 109

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    6 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    You should note that there is no shovel with the price of 0.

    I got stuck there too. Costed 4 submissions and a whole fortune of time.

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6 years ago, # |
  Vote: I like it +1 Vote: I do not like it

how to solve C? my recursive solution fails on pretest 5 :|

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    6 years ago, # ^ |
      Vote: I like it +1 Vote: I do not like it

    Make cases for each remainder of n with 4.

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    6 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    If you want code and some help, you can have a look here.

    Code — https://paste.ubuntu.com/26200030/

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    6 years ago, # ^ |
    ← Rev. 2   Vote: I like it 0 Vote: I do not like it

    I just found a segment of numbers which sum to (sum_of_sequence) / 2

    Not sure why it works, and if it's correct(did pass the tests though)

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    6 years ago, # ^ |
    ← Rev. 2   Vote: I like it +1 Vote: I do not like it

    I just got my solution for problem C accepted with only linear loops (O(N) time complexity), but my approach looked pretty weird I think. I did think of modulo 4 but I didn't write code that way. Can anyone suggest or show me proof of correctness? http://codeforces.com/contest/899/submission/33346265

    Get the sum of the first M elements. Get the absolute difference of it and the sum of the rest. M does not exceed N/2.

    Get the sum of the last M elements. Get the absolute difference of it and the sum of the rest. M does not exceed N/2.

    Get the sum of the middle M elements. Get the absolute difference of it and the sum of the rest. M = {N/2 - 1, N/2, N/2 + 1}.

    I wrote a little code brute-forcing all the cases and notice that with some N there exist solutions where the middle elements are chosen, and with some other N there exist solutions where the first (or last) elements are chosen.

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6 years ago, # |
  Vote: I like it +9 Vote: I do not like it

Any hint on what's 7th pretest in E?

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    6 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    I got it wrong because I wasn't recalculating the lengrhs correctly after merging, if that helps.

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    6 years ago, # ^ |
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    I think something like 13 1 1 1 2 2 2 3 3 1 1 2 2 1

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    6 years ago, # ^ |
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    try this case:- 1 3 3 3 2 2 3 3 3 1

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

    I have this one that is easier to debug:

    7 1 1 2 2 2 1 1

    Answe should be 2.

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6 years ago, # |
← Rev. 2   Vote: I like it 0 Vote: I do not like it

in the end of contest, i tried open hack attempt page but it not opened two min

so sad..

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6 years ago, # |
  Vote: I like it +7 Vote: I do not like it

Another nice round. It's harder and suits Div.2 better though. Cheers. ;)

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6 years ago, # |
  Vote: I like it +9 Vote: I do not like it

Problem D. Shovel Sale ********* Note that it is possible that the largest number of 9s at the end is 0, then you should count all such variants. **** if they neither give n<5 in pretest and nor in announcement then standing will be completely different.

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    6 years ago, # ^ |
    ← Rev. 2   Vote: I like it +1 Vote: I do not like it

    They did give n < 5 in the pretests, because I failed on pretest 7 before checking this condition.

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    6 years ago, # ^ |
      Vote: I like it +1 Vote: I do not like it

    They did give n < 5 on Pretest 6. I realised, two Runtime Errors and one WA later.

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    6 years ago, # ^ |
      Vote: I like it +1 Vote: I do not like it

    I'm sure n < 5 was given, one of my submissions hardcoded the values for n < 5 incorrectly and it worked once i fixed that.

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

    If n=4? Please tell!

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      6 years ago, # ^ |
        Vote: I like it +1 Vote: I do not like it

      answer will be 6. 1 2, 1 3, 1 4, 2 3 ,2 4 , 3 4.

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        6 years ago, # ^ |
          Vote: I like it +11 Vote: I do not like it

        Shucks I didn't consider this. I was like n<=4. cout<<0<<endl; Stupid me. :/

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          6 years ago, # ^ |
            Vote: I like it 0 Vote: I do not like it

          I did that too, but got a WA. Then corrected it for an AC pending systests

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        6 years ago, # ^ |
        ← Rev. 2   Vote: I like it 0 Vote: I do not like it

        Exxxxxxx! I didn't understand! **** Note that it is possible that the largest number of 9s at the end is 0, then you should count all such variants ****

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

      If N < 5 then all pairs making up sum ending with no 9 are counted. Refer to Enigma27's comment. Right at 5 the answer will be 1 since there exists one pair (4, 5) making up the sum 9 having one trailing 9. You can hardcode the case N < 5.

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6 years ago, # |
  Vote: I like it -36 Vote: I do not like it

i didn't like the recent contests on this site :)

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6 years ago, # |
  Vote: I like it +1 Vote: I do not like it

What was your Hack test case for B?

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6 years ago, # |
← Rev. 2   Vote: I like it +15 Vote: I do not like it

Thanks a lot for the suitable contest time. Due to the inconvenience of the time zone, it's really hard for me to find a CF contest to enter when I am not sleepy...

// Although today I realized contests wouldn't become easier for me when I'm awake :(

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

Hints Clues Solutions anything for D?

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6 years ago, # |
← Rev. 3   Vote: I like it +40 Vote: I do not like it

The system testing is starting now, but upsolving, virtual participation and viewing solutions and tests will be disabled till the end of the olympiad in Saratov (around 2-3 hours from now). Hope for your understanding.

UPD: Everything is up now.

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6 years ago, # |
  Vote: I like it +43 Vote: I do not like it

I noticed the lack of graph, DP, combinatoric and geometry problem in this year Olympiad.

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6 years ago, # |
  Vote: I like it 0 Vote: I do not like it

what is C pretest 5?

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6 years ago, # |
  Vote: I like it -9 Vote: I do not like it

DIV2 A IS SO HARD TO UNDERSTAND PROPERLY DUDE!

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6 years ago, # |
← Rev. 2   Vote: I like it 0 Vote: I do not like it

Wrote a simple stack solution for E after contest. I would be so suprised if it passes, but I dont know why it shouldnt pass. Is it possible that E is this easy? Edit: No its not, I didnt real statement carefully lol.

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    6 years ago, # ^ |
    ← Rev. 2   Vote: I like it 0 Vote: I do not like it

    I think that order of removing matters, without handling that I got wa on pretest 7.

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      6 years ago, # ^ |
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      Order does matter, but I dont think it will be problem in my solution. We will see soon.

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6 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Everybody in Russia uses Gregorian calendar. In this calendar there are 31 days in January, 28 or 29 days in February (depending on whether the year is leap or not)... People like me have no idea that leap year means 29 days in Feb....

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6 years ago, # |
  Vote: I like it 0 Vote: I do not like it

when wil be rating changes

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6 years ago, # |
← Rev. 2   Vote: I like it +1 Vote: I do not like it

How to solve C without knapsack?

My approach: if n=2, just like pretest. else:

1.Put n in set 1 and n-1 in set 2.

2.For each other element (n-2,n-3,...,1), just check (in descendig order) if it's better that the element goes to set 1 or 2.

How to prove that always works?

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    6 years ago, # ^ |
    ← Rev. 4   Vote: I like it 0 Vote: I do not like it

    I solved it using a simple approach.
    find the sum of integer from 1 to N.
    if sum is even number then answer will be 0 otherwise 1.
    in both the cases method of finding the numbers are same,do the half of the sum of 1 to N and store it in a variable called SUM.
    then start from (i=N ; i>=1 ; i--) and do this if (SUM — i >= 0) then subtract i from SUM and push_back i into you answer.
    you will get your answer vector.

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      6 years ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      I also thought of same logic but could not submit correct code in time . btw how do u prove your claim?

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        6 years ago, # ^ |
          Vote: I like it 0 Vote: I do not like it

        I wrote down some test cases on copy and checked it manually...
        you can think like this if you are subtracting a number from SUM and then your SUM becomes less than zero it means that you need to subtract a number which should be smaller than current number and i am going from N to 1 so it is sure that i will get that number.

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6 years ago, # |
← Rev. 2   Vote: I like it -38 Vote: I do not like it

I just wonder, if someone can tell me why I am disqualified?

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    6 years ago, # ^ |
      Vote: I like it +16 Vote: I do not like it

    probably because you cheated

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    6 years ago, # ^ |
      Vote: I like it +97 Vote: I do not like it

    Because you, Birjik, used fresh account to take part. It's ugly, unethical, disrespectful to me and the community.

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      6 years ago, # ^ |
      ← Rev. 2   Vote: I like it +1 Vote: I do not like it

      How did you know? There must be other users using fresh accounts too.

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      6 years ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      I wonder how did you detect his fresh account through his submissions?

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      6 years ago, # ^ |
        Vote: I like it +20 Vote: I do not like it

      First of all, it's not Birjik even though we do share same template code.

      Secondly, my real username is banned from CF (due to I cheated once a few years ago) and I gave you my sincerest apologies for that (several times), and asked to be unbanned in my real account.

      After several attempts, I even changed my PC and could somehow login and was eligible to participate in contests from my original account, but some times later, I couldn't even login into my account. After 5 mins of logging in, the site takes me out and I have to login again. Moreover, in most of the times I can't even register to the competition (due to my account was banned).

      I already noticed organizers about that issue. Didn't receive any reply. Okay.

      Well, so I decided to create my new account since I can't normally participate in competitions from my original account and this decision was made by me not because I do wanna cheat, I just don't have any options.

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6 years ago, # |
  Vote: I like it +1 Vote: I do not like it

How to solve problem-F?

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6 years ago, # |
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I saw some submissions for D with Digit — DP. Can someone explain their solution using Digit — DP ?

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6 years ago, # |
  Vote: I like it 0 Vote: I do not like it

for problem C: I noticed that the difference between every two adjacent numbers is 1. So if n is even, there will be two situations:if n/2 is even, answer is 0, or answer will be 1; Similarly, if n is odd, there are also two situations:if n/2 is even, answer is 1, or answer will be 0;

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6 years ago, # |
  Vote: I like it +27 Vote: I do not like it

Good contest, the problemset was perfect. We want more contests like this.

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6 years ago, # |
  Vote: I like it +26 Vote: I do not like it

Hi,MikeMirzayanov,can you tell me why I got disqualified? I didn't share code with anyone else.

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    6 years ago, # ^ |
    ← Rev. 2   Vote: I like it +48 Vote: I do not like it

    Because you created fake account to participate bro. It's ugly, unethical, disrespectful to me and the community.

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    6 years ago, # ^ |
    ← Rev. 9   Vote: I like it +43 Vote: I do not like it

    Because you (maybe a group of people, and I'm not sure who) used a fresh account to take part. It's ugly, unethical, disrespectful to me and the community.

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    6 years ago, # ^ |
      Vote: I like it +26 Vote: I do not like it

    I want to know why I got disqualified,too.I didn't share code with anyone else,either.

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      6 years ago, # ^ |
        Vote: I like it +39 Vote: I do not like it

      My friend eselppa got disqualified in yesterday's contest too, he solved all problems by himself, he didn't cheat, I can prove it. The reason why this account's ID is very similar with mine is that he wants to play a joke to me. That's not my account.

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        6 years ago, # ^ |
        ← Rev. 2   Vote: I like it +38 Vote: I do not like it

        But he used fresh account to take part. It's ugly, unethical, disrespectful to me and the community.

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          6 years ago, # ^ |
          ← Rev. 2   Vote: I like it +13 Vote: I do not like it

          Didn't you also use a fresh account to take part when you first participated. It's ugly, unethical, disrespectful to me and the community.

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    6 years ago, # ^ |
    ← Rev. 2   Vote: I like it +53 Vote: I do not like it

    I found a new problem ... How many "It's ugly, unethical, disrespectful to me and the community." in the replies of this comment. since the answer may be very large print it module 10^9+7.

    UPD : You also have to consider the occurrences in the replies after this reply.

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      6 years ago, # ^ |
        Vote: I like it +17 Vote: I do not like it

      Stop making jokes about cheating, it's ugly, unethical, disrespectful to me and the community.

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      6 years ago, # ^ |
        Vote: I like it +19 Vote: I do not like it

      Good god how did this become a memeful copy-pasta so fast dude. I mean it's ugly, unethical, disrespectful to m-

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6 years ago, # |
  Vote: I like it +1 Vote: I do not like it

Why I can't submit now?? The contest is over and the system testing has finished ...

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6 years ago, # |
  Vote: I like it +1 Vote: I do not like it

hello in your contest in b problem you gave test cases that there is two contiguous leap years but its impossible in real (one of them is test case 13) please correct it

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

    Leap years are ones with 29 days in February not 28.

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      6 years ago, # ^ |
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      my bad :(

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        6 years ago, # ^ |
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        Look at all the people who failed problem B. They all did same mistake as you. It's because of the problem statement, January, 28 or 29 days in February (depending on whether the year is leap or not) , which gives an idea that correspondly leap has 28 days and not leap has 29 days.

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6 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Hello, I found a very weird bug in my code for problem E. It works fine for many test cases except for the case when n = 2 and the 2 numbers are distinct. I checked on my local machine and it gives the error.

*** Error in `./a.out': munmap_chunk(): invalid pointer: 0x00005613cbdd2788 ***

Checking on gdb, I find that the pointer majt remains the same as mait, even though I'm doing majt--; which should make it ma.end() if we are removing the first number (Note that this works for other test cases, i.e. n!=2, even for n=1). I'm unable to figure out why. Any help would be appreciated. Thank you.

Code ID: 33351982

2
1 2
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6 years ago, # |
  Vote: I like it +16 Vote: I do not like it

Well,the time is best for Chinese!We always had to stay up late for the CF round :(

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6 years ago, # |
  Vote: I like it +22 Vote: I do not like it

Why are submissions not being judged?? I submitted 4 hours ago and it still writes "in queue" as status. :/

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6 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Why crash?

http://codeforces.com/contest/899/submission/33344155

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    6 years ago, # ^ |
      Vote: I like it +5 Vote: I do not like it

    You are erasing from set at the same time you are iterating over it. So when you delete an element the inner structure changes. You should create a temporal set and erase from it

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6 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Wrong answer on test 7 on problem F. Anyone have any idea? I am unable to find my bug. I have tested several cases but couldn't find anything.

Here is my submission 33357864

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6 years ago, # |
← Rev. 2   Vote: I like it -10 Vote: I do not like it

[Deleted] I post it on the wrong topic.Sorry.

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6 years ago, # |
  Vote: I like it 0 Vote: I do not like it

What's problem with this E code? 33382017

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

Hi,MikeMirzayanov,can you tell me why I got disqualified? I didn't share code with anyone else.Please give me a reason.

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    6 years ago, # ^ |
      Vote: I like it +23 Vote: I do not like it

    Because you, czr, used fresh account to take part. It's ugly, unethical, disrespectful to me and the community.

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

      Oh, you're really ugly, unethical, disrespectful to me and the community. As far as I'm concerned,you,it's your third fresh account! You should be banned by MikeMirzayanov in no time. And I can't imagine a more ugly and unethical behaviour than using a fresh account to point out that I used a fresh account!

      What's more,I do not use a fresh account and it's the first time I participated in a CF Contest!

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6 years ago, # |
  Vote: I like it 0 Vote: I do not like it

I've been stuck for ages... anyone know what test #33 was on div2E? http://codeforces.com/contest/899/submission/33345602

My answer is 1 + the correct answer. :(

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