snowysecret's blog

Codeforces Round #761 (Div. 2)
By snowysecret, 2 years ago, In English

Hello, Codeforces!

I am happy to invite you to Codeforces Round 761 (Div. 2), which will take place on 16.12.2021 16:35 (Московское время). The round will be rated for participants with rating lower than 2100. Notice the unusual starting time.

All the problems were authored and prepared by me. The round wouldn’t be possible without these people:

You will have 2 hours to solve 5 problems, one of which is divided into two subtasks. One of the problems is interactive, please see the guide of interactive problems if you are not familiar with it.

Wish you good luck and high ratings!

Here is the score distribution: $$$750-1000-1500-(2000-1000)-3000$$$.

The editorial is here: click.

UPD: Sorry but there is a checker bug in problem E. All submissions of problem E will be rejudged soon.

UPD2: Rejudge done, the round remains rated.

Congratulations to the winners (will be updated after system tests):

Congrats!
  • Vote: I like it
  • +773
  • Vote: I do not like it

Comments Comments (152)
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2 years ago, # |
  Vote: I like it +449 Vote: I do not like it

As a tester, I can confirm that the contest does indeed contain problems.

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2 years ago, # |
  Vote: I like it +165 Vote: I do not like it

As a tester, I cannot participate. :(

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2 years ago, # |
  Vote: I like it +50 Vote: I do not like it

As a tester, I would recommend everyone to read all problems

Good luck and have fun :D

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2 years ago, # |
  Vote: I like it +67 Vote: I do not like it

As a tester, snowysecret orz

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2 years ago, # |
← Rev. 3   Vote: I like it +41 Vote: I do not like it

As a tester, I can reassure you the tasks are pog. Highly recommend everyone to register. I can also confirm that the first question is QA.

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2 years ago, # |
  Vote: I like it +65 Vote: I do not like it

As a tester, I can say the problem statements are short, so I recommend everyone to join!

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2 years ago, # |
  Vote: I like it +182 Vote: I do not like it

As a tester, I sometimes test CF rounds.

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2 years ago, # |
  Vote: I like it -16 Vote: I do not like it

Hopefully interactive problem isn't divided into subtasks.

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

Looking forward to tomorrow's round! Finally a 5-problem div2 :)

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2 years ago, # |
  Vote: I like it -34 Vote: I do not like it

yet another unusual time

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    2 years ago, # ^ |
      Vote: I like it +80 Vote: I do not like it

    Unusually usual unusual time

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      2 years ago, # ^ |
        Vote: I like it +10 Vote: I do not like it

      Why does this make sense? XD

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        2 years ago, # ^ |
          Vote: I like it +33 Vote: I do not like it

        Normally, one would expect “unusual time” to usually be unusual, yet, if you look at the times of some of the recent contests, it is actually pretty usual, at least more usual than what one would usually expect, which is unusual. Thus, the unusual time can be considered to be unusually usual.

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2 years ago, # |
  Vote: I like it +49 Vote: I do not like it

As a reader, I can see that there are 3 times more testers than problems. :)

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

blue forces

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2 years ago, # |
  Vote: I like it -26 Vote: I do not like it

how easy it is to increase contribution "as a tester";

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    2 years ago, # ^ |
      Vote: I like it +24 Vote: I do not like it

    You know ,How difficult it is to test a problem "as a tester"?

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

it's time to become cyan (tyan)

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

Suprisingly, the contest is set to the Kazakhstan's Independence Day(16 Dec). Moreover, it is 30th anniversary of our independence.

GL&HF!

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2 years ago, # |
  Vote: I like it -10 Vote: I do not like it

would love it if Problem E is the nice interactive problem without subtasks.

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

After a long time, finally an interactive problem !

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

As a participant, I am excited to participate in the contest XD.

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2 years ago, # |
  Vote: I like it +8 Vote: I do not like it

as a participate ,i can confirm that i will lose my rating

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

    you have no rating to lose xD

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      2 years ago, # ^ |
        Vote: I like it +24 Vote: I do not like it

      as a participate ,,i can confirm to you that you are not funny XD

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

      A legendary grandmaster can say the same thing to you too ...

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        2 years ago, # ^ |
          Vote: I like it -40 Vote: I do not like it

        go play with your div3

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

          Okay play div2 with me then and give them some time to get better bruh ,but don't bully mate:)

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

      I assure you, bullying is bad for your health..

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

What's the rule of contests proposing now? IIRC I think only orange or author in previous rounds can propose contests now, it this rule changed again? >_<

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2 years ago, # |
  Vote: I like it +8 Vote: I do not like it

Congratulations on your first CF round as a problem setter. Hoping for a good round and some good problems.

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

No disrespect intended, but I'm very curious to how a mid-low expert can come up with problems that sufficiently challenge a 2100 rated person. I guess I'll find out tomorrow.

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    2 years ago, # ^ |
      Vote: I like it +61 Vote: I do not like it

    Rating, apart from estimating one's skills and knowledge, at the very least also heavily depends on speed and ability to handle stress.

    Look up Danny Sleator, he has a cyan rating here FYI, but I bet you wouldn't doubt his knowledge or ability to create a problemset that'd easily make the vast majority of competitors look pathetic.

    "Knowing" or "being able to" is not enough, you need speed, this is also why this thing we're doing is called competitive programming. Good luck tomorrow!

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    2 years ago, # ^ |
      Vote: I like it +44 Vote: I do not like it

    Given enough time, (some) experts can solve hard (<=2600R) problems and when you make the problem you have plenty of time to think of a solution for it before proposing. Also, when people usually make problems they have a vague idea of a possible solution in mind, which makes solving it way easier compared to a person who just read the problem and has to solve it in a 2h period.

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

    Guess you've found out now:)

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

Small question: If there is anyone who proposed a round in the past, how long is it from when you last set it public to the day it got approved? Is it 3 months or longer?

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

Hope I can reach expert after this round :D

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2 years ago, # |
  Vote: I like it +14 Vote: I do not like it

so nitin_05 and ashokesen02 both registered in this round.

  1. will they cheat again in this round?
  2. if so, will they not be caught and be rated again?!
  3. who will cheat better and drain more ratings from other innocent users?!?
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    2 years ago, # ^ |
      Vote: I like it +3 Vote: I do not like it

    Get a life. Every time i see you spamming these names. I am not supporting cheating but you can not stop someone from cheating.

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

As a beginner I can confirm that the contest is gonna be hard for me. But I am hopeful.

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

I wish good luck everyone!

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2 years ago, # |
  Vote: I like it -12 Vote: I do not like it

According to the distribution: $$$ 750−1000−1500−(2000−1000)−3000 $$$, is D1 harder than D2?

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

    Can someone please explain then?

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

      I can't be sure but I think if someone is able to solve D1, a small optimization will solve D2 too. So coming up with an approach for D1 will be difficult than solving D2 after you have solved D1.

      But I would suggest don't make any assumptions from this score distribution, check number of solves during the contest.

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

      You can think of a question D with 3000 points split into two parts.

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2 years ago, # |
  Vote: I like it -31 Vote: I do not like it

As a jia xin tang,ran ran ni dai wo zou ba!

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

Is the problem score related to the difficulty level of the problem?

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

It's time to become purple !!!

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

Let me get to the blues(^.^) UPD: Missed by 1 rating points, Sed life

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

Can anyone explain what does this line mean — "The jury is adaptive, which means the indices of impostors may not be fixed beforehand and can depend on your questions. It is guaranteed that there is at least one set of impostors which fulfills the constraints and the answers to your questions at any time."? Does this mean that the indices may change as we ask questions?

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

    You must ask in "ask a question" section, for any doubts during the contest time.

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

    I think it means test case are different base on your query.

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2 years ago, # |
  Vote: I like it +49 Vote: I do not like it

As you know, in codeforces, blue is stronger than red.

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

I have a question: If I didn't register on the round, didn't send ny codes, but during the contest took a glance at the results board and the problems, my rating won't change right? I'm just too worried

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

    Rating won't change unless you make a submission

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

    Unless you didn't send any code on the contest, there will be no change in your rating. I remember registering on some contests and I didn't send any code, but my rating hadn't changed.

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2 years ago, # |
← Rev. 2   Vote: I like it -61 Vote: I do not like it

.

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

Nice problem set!

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    2 years ago, # ^ |
    ← Rev. 2   Vote: I like it -95 Vote: I do not like it

    Yeah, really nice, top 10 worst div.2 I've seen.

    A: Boring implementation.
    B: Classical Problem, I've seen it >5 times on cf.
    C: The only common problem.
    D: It seems that the interactor is a joke, and the problem is a joke as well.
    E: Maybe to hard for div.2E, my 2800 friend can't solve it as well

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      2 years ago, # ^ |
        Vote: I like it +30 Vote: I do not like it

      The observations used in 2E are not advanced at all, and is very cool for its position. So it's not too hard for div2. Last problems of div2 typically get 50 solves which is the case here too.

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

        OK I read the editorial, now I'm more embarrassing

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      2 years ago, # ^ |
        Vote: I like it +50 Vote: I do not like it

      I solved E. It's quite interesting. How did your friend get 2800?

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2 years ago, # |
  Vote: I like it -51 Vote: I do not like it

Any hints for C? I had some thoughts with greedy algorithms but I always can find counterexamples. Can it be solved greedily? Or at least what's the main idea?...

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

    You’re not supposed to ask for hints about the problems during the contest, you can read the rules for more information. If you really want to know how to solve the problem, you can wait for the editorial for the contest, which will be out soon;)

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

    Try printing n % i for 1 <= i <= n and see if you can make an observation

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

    Yes, it can be solved greedily.

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2 years ago, # |
  Vote: I like it +12 Vote: I do not like it

awesome problem E! [tho TL is a bit tight]

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

Going to be Blue Again.. :D

Nice Problemset Sir !! Thank you for the contest <3

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2 years ago, # |
← Rev. 2   Vote: I like it +77 Vote: I do not like it

I wasted 30 minutes on D1 because I had mistakenly assumed that 0 meant crewmates and 1 meant imposters.

I am not just a clown. I am the entire circus.

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

    DAMN

    Damn bro, me too. Can I join the circus with you?

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

    wasted 40 min and managed to figure out in the last 30 seconds of the contest

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

    third impostor

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

Hint for B.

:)

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

The entire contest was casework...

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

Great round! I like the problems!

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    2 years ago, # ^ |
      Vote: I like it +8 Vote: I do not like it

    I told you before the contest XD, like even if its not the easiest div-2 rounds, the tasks are still very interesting and elegant (Go read editorial)

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2 years ago, # |
  Vote: I like it +36 Vote: I do not like it

Problem D (especially D2) is really really nice.

The $$$\frac{4n}{3} + 6$$$ and $$$n + 6$$$ observations are really wow moments when you get them.

Unfortunately I got stuck on $$$\frac{4n}{3}$$$ to $$$n$$$ optimization for too long to code it before the contest ended T_T.

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

Thank you for the contest! I like it a lot~~

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

E is really nice

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

( >︹<) AMOGUS ( >︹<) IM JUST IRONIC BTW ( >︹<) IM NOT ACTUALLY AMONG US NORMIE

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2 years ago, # |
  Vote: I like it +8 Vote: I do not like it

Really curious to know how the adaptive interactor for problem D1 and D2 works?

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    2 years ago, # ^ |
      Vote: I like it +8 Vote: I do not like it

    I think the main principle of the adaptive interactor is to fail any solution that asked wrong queries and cannot figure out the answer surely. I don't know what algorithms are used, but if it detects it can fail the solution, it will find the imposter sequence and do it.

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      2 years ago, # ^ |
        Vote: I like it +8 Vote: I do not like it

      As a tester, I can give some insights:

      Unfortunately, there isn't any protocol we know of that definitely eliminates random solutions. The interactor here relies on a combination of anti-random heuristics and exhaustively trying small tests to root out random solutions.

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

How to solve D?

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

Very boring game!!!

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

The observation of D2 is pretty good, but after that it is just boring implementation. Glad I am just above div2 so I don't have to force myself to write it down.

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2 years ago, # |
← Rev. 3   Vote: I like it 0 Vote: I do not like it

For B problem , my solution is wrong , why?

Firstly , I enumerate the factors of $$$n$$$ , assuming the factor of $$$n$$$ is $$$x$$$.

Assuming that $$$a = μx , b = λx , c = x$$$ , and $$$ \gcd(λ,μ) = 1$$$. So this is accord with the second condition.

Then through the first condition , we know that $$$μx + λx + x = n$$$

so $$$μ + λ = \frac{n}{x}-1$$$

so when $$$(\frac{n}{x}-1)$$$ is ODD ， we let $$$μ=\lfloor(\frac{n}{x}-1) / 2\rfloor$$$ , $$$λ = μ+1$$$ , then we got the final answer , which is $$$O(T \sqrt{n})$$$

WHY THIS IS WRONG ANSWER ? PLEASE HELP ME.

sorry for my poor english!

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

    Assume n = 15. Divisors of 15 are x = 1, x = 3, x = 15 and x = 5. All of them are odd, so n/x will always be odd too. So, n/x-1 will be even. You haven't described this case if I understood you right.

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2 years ago, # |
  Vote: I like it +7 Vote: I do not like it
Spoiler
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2 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Apart from the tricky problem statement in D, very nice contest! Kudos to the creator!

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

Thank you for the nice problems!

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

The jury is adaptive, which means the indices of impostors may not be fixed beforehand and can depend on your questions.

So if you have n = 6 and ask queries as in D1 editorial (1, 2, 3), (2, 3, 4), (3, 4, 5), (4, 5, 6), couldn't the imposters just always go to the positions the query asks, so there all the answers to the queries will be same?

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

    "It is guaranteed that there is at least one set of impostors which fulfills the constraints and the answers to your questions at any time."

    This means that there must be a set of indices consistent with all your queries so far.

    So for all your queries to return imposters, there must be imposters as the majority in the positions of all queries, for which the smallest set we can do is $$${2, 3, 4, 5}$$$ which violates $$$\frac{n}{3} \lt k \lt \frac{2n}{3}$$$. So its impossible to generate a counter-case under the given constraints which satisfies all your queries.

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

      ooh, thanks. my brain is kinda dead from waking up at 6am for advent of code lol.

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

great contest. I'm lucky that after I can't get idea of D1, I checked E and finished it in time.

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

( >︹<) AMOGUS ( >︹<) IM JUST IRONIC BTW ( >︹<) IM NOT ACTUALLY AMONG US NORMIE

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2 years ago, # |
  Vote: I like it -57 Vote: I do not like it

For D2, why the limit is n+6? My submission with total<=n+8 got WA and I spent long time making it to total<=n+4.

Why the limit so tight？I think n+20 it better.

If you really want the big constant factor got WA. Why not let the limit = n + 4，or you are stupid that you can't do with limit = n + 4?

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

Really good contest. Statements were clear and problems were interesting. Loved it!

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

any tips for interactive problems i become blank as soon as i saw them and dont know how to approach is there any good video resources if so plz share thnx in advance.

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

    Firstly, try to read the announcements before every contest. It was mentioned there that one of the problems is interactive along with a link to understand what it was and how to approach it. U can go to that link for understanding interactive problems.

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

Can anyone suggest me some problems like today's C?

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

I don't know why I wa1 on problem D. So sad.

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

    Your queries can contain same players in them. You can try 6 players with 345 as impostors for example.

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

Nice contest! Although I will be getting a negative delta, still I liked the contest.

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

Problemset was awesome !

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2 years ago, # |
  Vote: I like it +42 Vote: I do not like it

What a cool contest snowysecret made for today. There would be more such wonderful contests. Most of all I liked problems D1 and D2. Although unlikely I would solve D2. But the D1 problem was quite solvable for me. But of course it's a pity that I could'n solve it. And problem C was also cool. In one sentence, I would like to say, thank you snowysecret for such a wonderful contest.

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

can someone please tell why this is giving me RTE on the first test case? it is working fine on my compiler https://codeforces.com/contest/1617/submission/139558421

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

    In queries t1,t2,t3, a can be equal to b+1 or vice versa, then in that case query(a,a+1,b) or similar queries will be wrong as all indices won't be distinct.

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

nice problems , had fun in this contest

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

What's the meaning of "Wrong question format" in D2, I WA many time on it;

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

    Some possibilities that can cause "Wrong question format":

    • The first character is not ? or !

    • The first character is ?, and the three indices outputted are not distinct, or not in the range $$$[1, n]$$$

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

C can be solved in linear time 139510468

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

nyaharo

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