ipaljak's blog

CEOI 2021 Mirror
By ipaljak, history, 21 month(s) ago, In English

Hi all,

The Central European Olympiad in Informatics (CEOI) 2021 is starting tomorrow and we'd like to invite you to participate in two online mirror contests that will feature the same problems as the two competition days.

The day 1 mirror will take place on Thursday, September 2nd, 11:00 CEST, and day 2 mirror will take place on Saturday, September 4th, 11:00 CEST. Both contests will last for 5 hours, contain 3 IOI-style problems, and have full-feedback.

In order to participate in the CEOI 2021 Online Mirror contest, you'll need an account on evaluator.hsin.hr. After logging in, you can register for the Mirror contests on the "Events" page.

Hope you'll enjoy the contests.

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21 month(s) ago, # |
  Vote: I like it -46 Vote: I do not like it

I can determine the level of Russian Olympiads. Definitely, good level of question will be there to check our problem-solving skills.

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    21 month(s) ago, # ^ |
      Vote: I like it +35 Vote: I do not like it

    How is this olympiad related to russian ones?

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      21 month(s) ago, # ^ |
        Vote: I like it -7 Vote: I do not like it

      My apology, I thought it is a Russian one.

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21 month(s) ago, # |
  Vote: I like it +68 Vote: I do not like it

vito1036 animal will get gold medal

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21 month(s) ago, # |
← Rev. 2   Vote: I like it +26 Vote: I do not like it

Hi, i'm trying to register in Mirror #1 but i can only see Mirror #2 on "Events" page. Does that means registration is closed already?

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21 month(s) ago, # |
  Vote: I like it +29 Vote: I do not like it

We will reopen the registration and leave it open until 10 min before the contest, please register earlier for day 2.

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    21 month(s) ago, # ^ |
    ← Rev. 2   Vote: I like it +10 Vote: I do not like it

    Some students of the Israeli team forgot to register early for day 2 :( (on day 1 they could register late but wasn't aware that this was a one time thing) Is there any chance you can open late registration today as well? ipaljak

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21 month(s) ago, # |
  Vote: I like it 0 Vote: I do not like it

It has been 5 minutes, but I can't see the problem statements. Where are they?

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    21 month(s) ago, # ^ |
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    Just noticed that they are not under each task but one full pdf placed under "Attachments" on the contest page.

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21 month(s) ago, # |
  Vote: I like it 0 Vote: I do not like it

Where we can find problems, if not registered for contest?

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21 month(s) ago, # |
  Vote: I like it +16 Vote: I do not like it

Where is the ranking for Mirror Contest ?

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21 month(s) ago, # |
  Vote: I like it +3 Vote: I do not like it

sadly due to some circumstances, I couldn't enter the contest in its time so is there any way that i can take virtual participation?

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21 month(s) ago, # |
  Vote: I like it +18 Vote: I do not like it

Does anyone have the link to scoreboards?

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21 month(s) ago, # |
  Vote: I like it +7 Vote: I do not like it

The test data for subtask 4 of Diversity is weak, I somehow passed that but failed 2 and 3.

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    21 month(s) ago, # ^ |
      Vote: I like it -54 Vote: I do not like it

    Actually, i have a friend, who submitted fully incorrect solution, it didn't pass samples, but passed all subtasks from 1 to 5. I wonder, what they put there.

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      21 month(s) ago, # ^ |
        Vote: I like it +33 Vote: I do not like it

      In case you're not being sarcastic, the samples clearly have more than 1 query and a 64 point solution doesn't need to handle them.

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        21 month(s) ago, # ^ |
        ← Rev. 2   Vote: I like it +7 Vote: I do not like it

        I meant not multiquries, but answering 1 query. Let me tell you my solution. We make array cnt[], where cnt[a] is number of times it appears in given array. Then let’s sort it like: put the biggest element in the middle, then next biggest on the right, next on the left etc. Then calculate answer.

        My friend just sorted cnt array and calculate answer on it.

        I didn’t write clearly, why I was surprised. I believe you will understand me.

        PS: I don't know other solutions. If your solution is different from mine, share it please.

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          21 month(s) ago, # ^ |
            Vote: I like it +8 Vote: I do not like it

          Hmm, thing is i did just sort the cnt array and submit it, and I got 0 points, before sorting it properly and getting 64. And I dont think I had any bugs in the first version.

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          21 month(s) ago, # ^ |
            Vote: I like it +8 Vote: I do not like it

          My solution is the same as yours so if your friend's solution actually got 64 then it would be really strange. Would you mind asking him to share the code?

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          21 month(s) ago, # ^ |
            Vote: I like it 0 Vote: I do not like it

          My solution was the same, now I realise it probably failed because I used int instead of long long in one place. (In subtask 2 & 3 the number of species was small so the sizes of individual species were much bigger than in 4.)

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21 month(s) ago, # |
  Vote: I like it +19 Vote: I do not like it

Does anybody know where is the editorial of today?

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21 month(s) ago, # |
  Vote: I like it +14 Vote: I do not like it

Where can I submit now?

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21 month(s) ago, # |
  Vote: I like it +47 Vote: I do not like it

Is there any upsolving contest?

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21 month(s) ago, # |
  Vote: I like it +27 Vote: I do not like it

I found $$$O(N\sqrt{N}logn))$$$ solution for Diversity is there any other faster solution? Because I m not sure if it will pass the tests

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21 month(s) ago, # |
← Rev. 3   Vote: I like it +38 Vote: I do not like it

I solved this riddle day before the contest and was thinking about how to extend the solution for asking a problem about it but seems like someone was faster(Task: Newspapers, also I still don't how to extend the solution)

Spoiler
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21 month(s) ago, # |
  Vote: I like it +23 Vote: I do not like it

Task newspapers was nearly identical to an old problem from Csacademy. I hope it didn't affect the standings.

https://csacademy.com/contest/archive/task/catch-the-thief/

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    21 month(s) ago, # ^ |
    ← Rev. 2   Vote: I like it 0 Vote: I do not like it

    I found the same thing for half of the points but didn't pass the tests for 1. and 3. subtasks, could anybody help?

    Code

    Edit: The last part is not necessary. So I removed it.

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21 month(s) ago, # |
  Vote: I like it 0 Vote: I do not like it

I'm quite sure I registered for both Day 1 and Day 2 yesterday.
I checked the registration page but it seems I'm not registered. Does anyone have this issue?

Screenshot
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21 month(s) ago, # |
  Vote: I like it +10 Vote: I do not like it

Can someone upload the tasks here?

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    21 month(s) ago, # ^ |
    ← Rev. 2   Vote: I like it +10 Vote: I do not like it

    They're not available on the site yet, so I've uploaded them on google drive for now. I'll update this post once they've officially been posted. The tasks have been uploaded in the site.

    Day 1 tasks

    Day 2 tasks

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21 month(s) ago, # |
  Vote: I like it +10 Vote: I do not like it

Lol, everyone got baited by day 2 C.

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    21 month(s) ago, # ^ |
      Vote: I like it +12 Vote: I do not like it

    Great problem for the most difficult one on CEOI, shame no one got any points :(

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21 month(s) ago, # |
  Vote: I like it +8 Vote: I do not like it

Will it be possible to send solutions?

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21 month(s) ago, # |
← Rev. 2   Vote: I like it +8 Vote: I do not like it

Btw, I dont know why the limits on day 2 A were so small. I think I can make my solution work for n<=10^5, m<=10^9 . Did people have different solution to it ?

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    21 month(s) ago, # ^ |
      Vote: I like it +34 Vote: I do not like it

    I don't think there's any benefit in providing such tough constraints, the crux of the problem is essentially to solve it in polynomial time.

    If anything, small bounds on an interactive problem probably help the system testing (and, we can upperbound the number of interactions by $$$nm$$$, so even very bad solutions can't do too many turns).

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      21 month(s) ago, # ^ |
        Vote: I like it -22 Vote: I do not like it

      Well, that s your opinion. Also , I dont think it d require much more effort for system testing. I am mostly curious whether someone had a different approach to solve the problem, with dp or something .

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        21 month(s) ago, # ^ |
          Vote: I like it +7 Vote: I do not like it

        It's possible to solve it with dp[number of 1s][number of >1s].

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21 month(s) ago, # |
  Vote: I like it 0 Vote: I do not like it

Where can I submit now? ipaljak

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    21 month(s) ago, # ^ |
      Vote: I like it +21 Vote: I do not like it

    You can submit them now on evaluator.hsin.hr under Tasks -> Special Contests -> 2021. CEOI

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21 month(s) ago, # |
  Vote: I like it +24 Vote: I do not like it

Tasks, results and editorial are available here

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    21 month(s) ago, # ^ |
      Vote: I like it +27 Vote: I do not like it

    Could you please release the test data?

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21 month(s) ago, # |
  Vote: I like it +61 Vote: I do not like it

ipaljak jklepec could you please share the tests? Let's support ojuz. :((

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20 months ago, # |
  Vote: I like it +28 Vote: I do not like it

You can solve all problems except Stones here: Day 1, Day 2

How could the strategy of the interactor for Stones be? What should it print when it's in a losing position?

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