ch_egor's blog

Codeforces Round #657 (Div. 2, based on All-Russian olympiad in the name of Keldysh) [Rated]
By ch_egor, 4 years ago, translation, In English

Hi!

This Sunday will take place All-Russian olympiad for students of 5-8 grades, in the name of Keldysh. Good luck to all the participants! Olympiad is conducted under the guidance of the Moscow Olympiad Scientific Committee, in particular GlebsHP, ch_egor, Endagorion, vintage_Vlad_Makeev, Zlobober, meshanya, cdkrot, voidmax, grphil and, of course, Helen Andreeva.

We are happy to announce the Codeforces Round #657 (Vintage Codeforces Round #3) based on the problems of this olympiad! It will be a Div. 2 round, which will take place at Jul/19/2020 12:00 (Moscow time). You might have already participated in rounds based on the school olympiads, prepared by Moscow Olympiad Scientific Committee (rounds 327, 342, 345, 376, 401, 433, 441, 466, 469, 507, 516, 541, 545, 567, 583, 594, 622, 626) as well as vintage rounds (rounds 626 and 628).

The problems of this olympiad were prepared by DebNatkh, grphil, KiKoS, voidmax, I_love_myself, 300iq, isaf27 under the supervision of grphil.

Thanks ch_egor, vintage_Vlad_Makeev and meshanya for their help in organizing the Codeforces version of this contest and MikeMirzayanov for the Codeforces and Polygon.

Also I would like to thank the Tinkoff company and personally Tatyana TKolinkova Kolinkova for great help with organizing the competition.

Good luck!

UPD1: Scoring distribution: 500 — 750 — 1250 — 1500 — 2500 — (1500 + 1500)

UPD2: Editorial

UPD3: Winners!

Div. 2:

  1. purplesyringa
  2. yjw0203
  3. kaixinqi
  4. penguinman
  5. Frikkie

Div. 1 + Div. 2:

  1. antontrygubO_o
  2. risujiroh
  3. tmwilliamlin168
  4. sansen
  5. LayCurse
  • Vote: I like it
  • +236
  • Vote: I do not like it

Comments Comments (394)
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4 years ago, # |
  Vote: I like it +11 Vote: I do not like it

Number of problems?

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4 years ago, # |
← Rev. 2   Vote: I like it -86 Vote: I do not like it

im-back.jpg

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4 years ago, # |
  Vote: I like it -46 Vote: I do not like it

I am excited to see problems of Russian olympiad

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4 years ago, # |
← Rev. 2   Vote: I like it -138 Vote: I do not like it

.

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4 years ago, # |
← Rev. 4   Vote: I like it -61 Vote: I do not like it

[DELETED]

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4 years ago, # |
← Rev. 2   Vote: I like it -57 Vote: I do not like it

.

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4 years ago, # |
  Vote: I like it +177 Vote: I do not like it

The last time I saw these many reds was when I witnessed an accident.

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4 years ago, # |
  Vote: I like it +35 Vote: I do not like it

What are vintage rounds?

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

Can I be BLUE....

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4 years ago, # |
  Vote: I like it +737 Vote: I do not like it

No offence to anyone

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

    So true!

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

    Damn true. Even the school kids will solve till F and we would be struggling with C and D XD

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    4 years ago, # ^ |
      Vote: I like it +66 Vote: I do not like it

    Well I take offense

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

      Some big aim you had in your first year! Most desi Indians only think about hacking Facebook when starting in CS :P

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    4 years ago, # ^ |
      Vote: I like it +57 Vote: I do not like it

    Our education system needs improvement, If I would have started coding from my 5th grade then I think I would have been red coder till now, and I beleve its true for all Indians.

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      4 years ago, # ^ |
      ← Rev. 2   Vote: I like it -21 Vote: I do not like it

      Nah ! If i ask a random 5th grade Indian student to start cp ,he/she would not be even be excited or motivated about it. Exceptions do exist but i am talking with respect to general population . I was more interested in playing football and cricket at that age .

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

        but what they taught in schools can be changed to what we want to be taught depending upon your interests which will eventually help us in future

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

        Even I was more interested in playing cricket, but I do wonder if I got introduced to CP I may have latched on to it because I had a good interest in maths and aptitude at that age and also CP has a gaming front to it with all the rankings and points, you just need a given the proper introduction and some good peers to compete with :)

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

      Yeah many of us feel the same, but no probs curently CP is rapidly evolving in India. India will have good number of GM's after a decade or so. And we need to create awareness about CP for a 5th grade student.

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

      I remember learning paint in 5th grade in school lol

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

      If coding from (5th grade -> today) = red coder then coding from (today -> today + 10 years) = red coder too. On top of that at this stage one will need much lesser time maybe 4-5 good years is enough.

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

    The same happens in Peru, but with the 5th semester students. From the 1st to 4th semester they just use microsoft excel.

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

    I wanted to code when I was a kid but didn't know where to start

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

    Logged in, only to upvote you.

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

    Our tradition is to memorize maths to pass exams.

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

    Would gladly take this over 70 years of communism.

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

    So hard to accept our education system

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

      you can't blame it on education system ..... NO

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    4 years ago, # ^ |
      Vote: I like it +31 Vote: I do not like it

    If this contest was for 5-8 grade students, I feel like I am a new born baby.

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

    Can feel this bro :')

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4 years ago, # |
← Rev. 2   Vote: I like it -37 Vote: I do not like it

Schools are like:

  • Taught in Class: Div2 A

  • Solved for practice: Div2 B

  • Homework: Div2 C

  • Exams: Div2 D,E,F

credits: adzo261

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

why is that unusual time ?

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

    Because problems took from olympiad and for prevent leaking it, round will be in the same time as olympiad

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4 years ago, # |
  Vote: I like it +55 Vote: I do not like it

me in 8th grader : tell my friend to open page 69th of biology book where there is a picture of penis anatomy
russians 8th grader : solve div2 problems

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4 years ago, # |
  Vote: I like it -7 Vote: I do not like it

Will the round be rated ch_egor? Asking because its not stated in the blog post:)

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

great !! we have 300iq now ... I am excited !!

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

That's a lot of red handles

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

Why are there so many down-voted comments in this blog?

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

(Notice the unusual timing)

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

I think people should think before voting because even the comments which are not at all negative are downvoted heavily.

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

we will b enjoying this contest

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

This is 5 AM in America where I live

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

    Considering the lockdown, does the time even mean anything anymore?

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

      Yes, there's something called body clock, which when affected can cause various health problems

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

Score Distribution?

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4 years ago, # |
← Rev. 4   Vote: I like it +7 Vote: I do not like it

....

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

Any reason why start time is 09:00 UTC, but not 09:05

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

    Probably because it's going to be held in parallel with All-Russian olympiad. Also to avoid clash with Topcoder and Codechef rounds.

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

Hope the problem statements are clear and interesting xD

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

How many problems and Score Distribution?

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4 years ago, # |
  Vote: I like it +111 Vote: I do not like it

Unfortunately, the last 6 months we have seen this instead of the exciting confrontation between tourist and jqdai0815 :)

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

    Nice!!!

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

    Rating: users participated in recent 6 months

    Since his last contest was on Feb 17, either he'll take part in another contest or disappear from the global rating in a month.

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

    At least he's visiting the site which means he is still alive. :)

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

Never mind. :)

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4 years ago, # |
← Rev. 2   Vote: I like it +8 Vote: I do not like it

What's the Score Distribution? ch_egor

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

No Score Distribution and No. of Problems yet? ch_egor

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4 years ago, # |
  Vote: I like it +84 Vote: I do not like it

where scoring distribution and number of problems will be published?? ch_egor

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

Scoring distribution says it's gonna be SpeedForces.

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

the first round during my summer vacation lol

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

I'm sure some people will join 5 minutes late thinking that round will start at 14:35 IST. :v

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

Is this Div1?

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

    Seems so! Not able to TOUCH a question!!

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

      I think we really need the help of pavan putra hanuman to solve the questions in this contest Xd.

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4 years ago, # |
  Vote: I like it +81 Vote: I do not like it

There has been a small typo guys.. it is a Div 1 round..

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

one hand pushups were easier for me than this round's problems!

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

I've never seen such DIV 2 contest.

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4 years ago, # |
  Vote: I like it +45 Vote: I do not like it

Now I get why Russians dominate in competitive programming. :)

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4 years ago, # |
  Vote: I like it +82 Vote: I do not like it

This is for 5th-8th grades?? OKK

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

I wasn't able to enter codeforces for almost 20 minutes. Anyone else facing the same issue?

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    4 years ago, # ^ |
      Vote: I like it +47 Vote: I do not like it

    I entered code forces but couldn't think what to do for the past 40 minutes. Anyone facing this same issue??

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

    Good that you didnt enter brother! God saved you from a heart attack!

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

    Yea me too

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

Should have rather rushed Bombsite B. On a serious note, is this really what 7th Grade Russians study? That is really impressive, no wonder Russians are such good coders.

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4 years ago, # |
  Vote: I like it +21 Vote: I do not like it

Great div1 round!!!!!!!!!

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

2 minutes silence for all those who were too eager for the round to be rated.

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4 years ago, # |
  Vote: I like it +31 Vote: I do not like it

After walking through first 3 question:

How to unregister???

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

OK now I'm looking forward to the editorial after the contest.

I'm feeling numb :( Is it a Div.2 round or Div.1 Round?

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

    Round by Red round for Red . Screaming Red.

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

    Lul bruh, I have the exact same pattern of submissions, except I'll become specialist by today evening.

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4 years ago, # |
  Vote: I like it +18 Vote: I do not like it

This is meant for students of grades 5-8? Wow.

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

worst div2 i ever seen !!

upd: I said worst cause its seems more div1 than div2 but named div2.

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4 years ago, # |
  Vote: I like it +64 Vote: I do not like it

I am proud that I have participated in Div 1 round.

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

I was getting 504 server error during first 20 minutes. I am feeling blessed.

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

    Are you the lucky one?

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

    I was having the same issue for a very long time. I think this happened with many people and not just a few of us. There were more than 15k people who registered and on the scoreboard there are only 8000 who have attempted a single problem.

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

      I think this could have been because it was very early morning in the US, so lots registered but not many woke up that early.

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

    You can always use these lightweight websites m1.codeforces.com m2.codeforces.com m3.codeforces.com in such trouble of server in my case these websites work almost everytime in a live contest.

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

    glad to hear it wasn't only me xD

    I was afraid I was the only one

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

feels like DIV1 to me

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4 years ago, # |
  Vote: I like it +58 Vote: I do not like it

New technique to manage server load.

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4 years ago, # |
  Vote: I like it +175 Vote: I do not like it

This is Div 1 :(

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4 years ago, # |
  Vote: I like it +57 Vote: I do not like it

Are you trying to kill those poor 5-8 graders ?

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

    They might have completed the problemset by now. Russian kids are seriously genious.

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

No wonder why tourist is so perfect.

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4 years ago, # |
  Vote: I like it +41 Vote: I do not like it

is there any way to undo submission?

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4 years ago, # |
  Vote: I like it +38 Vote: I do not like it

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

Ah, it was good that my connection was lost during the first 30 minutes.

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4 years ago, # |
  Vote: I like it +91 Vote: I do not like it

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4 years ago, # |
  Vote: I like it +105 Vote: I do not like it

When you realize that this was actually a Div. 1 contest

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

Hell lot difficult contest!

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

Was this div 2? this was Div 1 ! Even more difficult , Div 0 maybe

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4 years ago, # |
← Rev. 2   Vote: I like it +18 Vote: I do not like it

I registered for the contest, I planned to participate in it, but I slept and couldn't wakeup on time. I was feeling bad but then i read the problems and number of submissions. Thank god, i didn't wake up on time! xD

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

You miss 100 percent of the shots you don't take.

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

why there is no extra registration?

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    4 years ago, # ^ |
      Vote: I like it +55 Vote: I do not like it

    so that no one else falls into this trap

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

this is my first ever contest....I havent even solved a single question

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

Can I see how many problems those 5th-8th graders solved? Might be a motivation for me or myth buster for many!!

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

Before reading the comments I thought I don't deserve to be yellow on CF...

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4 years ago, # |
  Vote: I like it +59 Vote: I do not like it

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

The the last four div2 contests (excluding the Global round):

654 — widely disliked

655 — good set but queueforces

91Edu — good set but major crash

657 — Mere mortals can't solve more than 2

(I agree I'm not of high enough calibre to say all of this but damn it a man needs to vent — I've been at 1750 for 10 contests and I couldn't solve a single one — damn pretests 2)

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    4 years ago, # ^ |
      Vote: I like it +25 Vote: I do not like it

    Try to concentrate more on developing problem solving skills than on rating.

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

    Finally, I found you another expert, that did 0. And you have a much higher rating than me. I am sorry but I feel better now. ;_;

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

    A hint : abacab*bacaba needs to Output "NO"

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4 years ago, # |
← Rev. 4   Vote: I like it +60 Vote: I do not like it

Me after not being able to solve a single problem

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

    can u translate it for us who doesn't know the language?

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4 years ago, # |
  Vote: I like it +46 Vote: I do not like it

My son was in 8 grade, Now I don't have a son...

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

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

So much implementation :(

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

Div 0 round

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

What is wrong with my A:87342260

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

    1 11 abac???caba, answer should be no as they want "abacaba" exactly once

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

      You are right, Thanks for the test. Does that mean that I just need to add some cases to my code? Or I will code another solution

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

What is the third testcase for C?

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4 years ago, # |
  Vote: I like it +53 Vote: I do not like it

It's a record. First time 20k participant in div — 1 round.

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4 years ago, # |
  Vote: I like it +181 Vote: I do not like it

New technique to manage server Load

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

How to solve C?

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

    We will only buy more than one flowers of only one type iterate to choose this and greedily take other flowers who have ai >= chosen bi

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

      I tried to do like this but it hasn't worked(

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

      I tried similar approach but couldn't get past test 5. What did you fix to pass that?

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

        I didn't consider if no ai > chosen b

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

          I did that too at the end, still failed :( Guess something is wrong with my implementation. Thanks.

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

          Oh I found my mistake, I didn't handle this case: Number of Ai >= b is already >= n, in that case, I shouldn't take this b.

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      4 years ago, # ^ |
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      my idea was similar but I implemented it wrongly I guess. Thanks for replying.

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      4 years ago, # ^ |
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      I did the same and got WA2 T_T

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4 years ago, # |
← Rev. 2   Vote: I like it +8 Vote: I do not like it

Probably it was the hardest was the A ever

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

I don't see why is it rated for me when I'm clearly not a div1 participant

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

any hint for problem C !

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

    you knew that you should take a[i] at least once to take b[i]. so, first sort all array A, then if we take a[i] once, then we have two options in the next step. take a[i + 1] or (b[i] for all remaining flowers), so think where you will know that you will take b[i] for all remaining flowers. i think this enough as hint. if u wanna more hint or there is anything ambiguous , tell me.

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

What's tc18 for F1?

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4 years ago, # |
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It is kind of funny I visit the CF blogs comment section to see memes.

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

this contest is for grade 5 to 8,OMG i can only imagine level of Russian kids

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4 years ago, # |
  Vote: I like it +56 Vote: I do not like it

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4 years ago, # |
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Problem A is not as easy as I though, it actually requires more implementation than usual :)

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4 years ago, # |
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How to solve C? I tried a greedy strategy but it is not always optimal, and I believe fixing it would require m^2.

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    4 years ago, # ^ |
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    Why greedy is not always optimal?

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    4 years ago, # ^ |
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    I think it is something related to convex hull optimization technique.

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      4 years ago, # ^ |
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      I don't think so! I believe, greedy will work! Even though, I couldn't reach up to a correct solution.

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

    Notice that you will only take b_i of one type in the entire process (if you could improve the answer by taking another b_k, you would just take all b_k as it would be better), and that it is optimal to take all a_j >= to this b_i before taking it and that you must take a_i before you can take b_i.

    Now you can just iterate over b_i in descending order and do something like two pointer to keep track of the sum of a_j >= b_i taken while maintaining a used array to see if you've taken a_i already when taking b_i, so answer will be max of (asum till now + (n — a_taken) * b[i]), minus b[i] plus a[i] if we haven't used it yet.

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

      I tried same thing. But, might have missed out something in the implementation! :(

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        4 years ago, # ^ |
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        Did you clear the used array? I got 2 WAs and wasted 10-15 mins thanks to that

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          4 years ago, # ^ |
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          Actually, I used 2 PQ's to implement this. Maybe, I missed something in the casework.

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      4 years ago, # ^ |
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      Ahh I see. I tried to implement it backwards (iterate over all a_i in descending order and find the best candidate for b, which I thought was O(1) since you can keep track of the max b already taken and not already taken, but the max is not always optimal. Nice solution!

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

It seems Div1 round for Div2 participants. Too Hard! ToT

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4 years ago, # |
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Counter case for taking times modulo m / 2 (and ignoring hours), sorting and finding largest range with distance of max m / 2 — k using two pointer in D?

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

    I kind of did the same thing. My solution was failing on this-
    4 24 4 2
    16 0
    17 1
    18 2
    19 3

    Try this

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

"Wrong answer on pretest 2"

5 words which summarize the contest

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4 years ago, # |
  Vote: I like it +35 Vote: I do not like it

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

Seems like old codeforces rounds are back. Kudos to the author , Nice educational problemset. I hope more of such rounds which teaches how to solve real algorithmic problems in future, A B C were absolutely fabulous and not as per the score given to them , B is much more worthy than 1000 also. Again Thank You for such a brilliant round.<3

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

    Yes true many people will complain about how bad the round was just because it was hard but the truth is it was really educational as even the implementation was hard and required debugging skills and the problems logic was also good.

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      4 years ago, # ^ |
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      Specialists are dumb

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        4 years ago, # ^ |
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        Please have a look at my graph before making assumption :P

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      4 years ago, # ^ |
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      yeaheverybody is crying but it was quite a challenging round and a nice change

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

Should I register for next DIV2 round or not?

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

    Never fear for ratings always assume that they are a sideffect of Leaninig. If losing rating hurts you then also its normal try not looking at standings during the contest and also not at all look at your colour, just believe that giving contest and one like this is making you a learn something new, everyday. and also training you on what you already have learnt. Especially do all those contests given in a list combined in the announcement above. These are some of nowadays rare contest problemsets and amazing. Once again do register and solve problems for fun.

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

My First Div 1 contest done !! -_-

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4 years ago, # |
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Can someone give a hint for D? Is it a ternary search?

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4 years ago, # |
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can anyone tell me what is wrong in this solution?? https://ideone.com/rajqqD. This solution is for Acacius and String.

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

    Buddy submissions won't be visible for 1st hour after the contest(check the announcement) better paste your formatted code here

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

All these comments and the round is still running? seriously guys!!!

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    4 years ago, # ^ |
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    what?

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    4 years ago, # ^ |
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    I think people didn't have anything else to do.

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      4 years ago, # ^ |
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      OK I understand the problems were harder than usual ( and much more interesting :D ) but admins should shout down comments during the contest, what if someone talks about his solution for one of the problems!!!

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4 years ago, # |
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How to solve B?

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

    Nvm

    Didn't understand the statement completely

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    4 years ago, # ^ |
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    Fix a and then try to find b, c.

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      4 years ago, # ^ |
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      did same still WA.

      My code
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        4 years ago, # ^ |
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        Actually you should check for both $$$rem = m$$$ % $$$a$$$ and $$$rem = (m + a - 1)$$$ % $$$a$$$

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

    You have $$$n*a + b - c = m => n*a = m + x$$$ where $$$ x = c-b $$$ and $$$ m+x \in [-maxr+m,m+maxr]$$$ where $$$maxr = r-l$$$ Iterate over a and do a binary search over $$$n$$$ and check if $$$n*a \in [-maxr+m,m+maxr]$$$

    So if you have $$$n$$$ and $$$a$$$ you can simply fix $$$b = l$$$ and calculate $$$c$$$.

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      4 years ago, # ^ |
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      No need for binary search, just find closest values to m you can get by taking positive amounts of a using division.

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

Thanks for the great contest.

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4 years ago, # |
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Too difficult for me...fighting!!!!

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

For problem D, I think the hour value of trains wasn't necessary.
Was it supposed to be solved by iterating on all "minute" values of trains as a candidate of $$$t$$$ and then finding how many minute values intersect the range $$$[t-k+1, t-1]$$$?
Ofcourse, this will have few cases to consider like $$$k=1$$$ or $$$t<k$$$ but was this the overall idea for D?

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

is it just me?? the codeforces were saying "can't connect right now" by the time contest was starting and after 5 or so minutes later it started. I submitted my C problem solution before 2 minutes but it keeps loading and loading after the contest I checked whether it submitted or not. But it wasn't submitted?

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

People before contest were asking — how many problems? Meanwhile ch_egor to himself : Actually there are two problems.

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4 years ago, # |
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How to solve C ? is there a greedy approach to it?

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

    optimal flowers choice is to take some flowers only once, and then take one flower and put it to the rest of the slots in bouquet.

    you have to iterate through that flower which is gonna be put to the rest of the slots and update the answer.

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4 years ago, # |
  Vote: I like it +28 Vote: I do not like it

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

Can anyone from Russia covert 5-8 grades to the equivalent level of American or Indian Education system, or at least tell me students of what age group study in Grade 5-8 in Russia?

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

Problem B seems easier than Problem A.

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4 years ago, # |
  Vote: I like it +58 Vote: I do not like it

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4 years ago, # |
  Vote: I like it +48 Vote: I do not like it

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

I misread the contest as DIV-2 and registered it. I didn't knew it was DIV-1

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

why my B solution is wrong?

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

    There are some cases that n*a have to be above m

    In this case for example

    7 8 13

    the answer will be

    7 7 8 , where n = 2 and 2 * 7 > 13

    and the m%i method won't work for these cases.

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

try this test case if you are getting WA on pretest 2, problem A.

1
12
abacab?cabac
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    4 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    My code says "NO", and I was still getting WA on pretest 2

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

      Try this test case for problem A

      1

      11

      ???????caba

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

        Yes ddddabacaba

        Is the output. I did consider the fact that changing from start and end can have different effects, So I checked in both.

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    4 years ago, # ^ |
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    Thanks for the test, but what is the true solution?

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

super difficult problem sets, but it was very interesting!

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

implementation-heavy round...

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4 years ago, # |
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How is the answer for the 2nd test case in problem C is 16.Shouldn't it be (5+4+5+4+5)=23

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    4 years ago, # ^ |
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    you can choose 5,4,3 only once according to the question

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      4 years ago, # ^ |
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      Where it was written in the problem that we can choose those values exactly once.I thought that whenever we switch from one flower to other flower then we can use the ai and whenever we use the same flower consecutively then only bi comes into account or I understood the problem wrong?

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        4 years ago, # ^ |
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        try reading these lines from ques again

        _ He knows that after receiving the first flower of the i-th type happiness of his wife increases by ai and after receiving each consecutive flower of this type her happiness increases by bi. That is, if among the chosen flowers there are xi>0 flowers of type i, his wife gets ai+(xi−1)⋅bi additional happiness (and if there are no flowers of type i, she gets nothing for this particular type)._

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          4 years ago, # ^ |
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          Ok I got it. I was confused by the consecutive part. Thought the value bi is added only when we take same number of flowers consecutively,that lead to the confusion of ai part.

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4 years ago, # |
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[Deleted]

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

Where i can get Russian students Textbook for Computer , Maths (class 5 — 8th) in English . How they prepare , any resource which they use ?? Please tell , (currently i am in university , but was unable to solve this contest).

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4 years ago, # |
  Vote: I like it +41 Vote: I do not like it

There are lots of people (me included), who find the problems to be too hard for Div2 round. Please don't be so dissatisfied about it, because:

  • Rating. The problems were harder for everybody. Everybody solved less problems than they expected to, and the rating changes will be generally the same.

  • It's ok to solve 0 problems in this round. It's not because you're bad, it's because the round is hard! Remember to check editorials and solve some of the problems after the round ends!

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

First time absolutely sure before System Tests that my solutions wouldn't fail because they didn't get accepted in the first place.

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4 years ago, # |
← Rev. 2   Vote: I like it +30 Vote: I do not like it
Spoiler
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4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

I feel injured

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4 years ago, # |
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Anyone solved C using Priority_Queue?? what approach you used to solve Problem C

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4 years ago, # |
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can use sweep line on Problem D. But I have no time to implement it.

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

For E, is there a more efficient construction than:

     o
    / \
   o   o
      / \
     o   o
        / \
       o   o
          / \
         o   etc.

I believe this should work for $$$2k + 3 \leq n$$$, but I'm not super sure...

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

    I solved E with this technique, but there is a special case: if n =9 and k=2, then the answer is NO.

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4 years ago, # |
  Vote: I like it +32 Vote: I do not like it

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

One word to describe today's contest for me — CHAOS.

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

I request Codeforces to make it unrated for those , who were not able to solve even single problem . I never thought a contest for 5-8th grade student will be so much tough .

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

    Very interesting idea. Also good idea is to make it unrated for people who are losing rating.

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

    You don't know how stressfull it was, and now predictor says imma get 93 rating increase. so i would never want it go unrated.

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

How to solve B ?? looked easy but was not.

My approach : I iterated from l to r and took the modulas and checked

for m % i != 0 is l + (i — m % i) <= r.

for m % i == 0 just print i

but this approach is wrong can anyone explain why ??

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

    CHoose a particular value of i between l to r as a and binary search from 1 to m for n. and choose any feasible numbers n,b,c which satisfy the equation. n*a+b-c=m

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

I couldn't access the codeforces site the moment this contest started and I can access again after the contest has ended. I had to use vpn to give the contest. Did something like this happen with anyone? Also, any solution?

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4 years ago, # |
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They should at least give a warning for such a contest.

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    4 years ago, # ^ |
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    But they thought we were as good as the Russians kids DD:

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

A historic moment! Everyone including the newbies were allowed to get a feel of DIV-1 contest :P

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

In problem D, what was the point of printing the indices of cancelled freight trains too? We are already printing minimum cancellations and the optimal starting time.

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

Test case for A: 1. abacab?bacaba -> NO 2. abac?b?bacaba -> YES -> abaczbabacaba

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

i solved prob A just 20 minutes and prob B 3 minutes before contest ended, there was some corner cases that i did't come up with during that time, in my opinion i really like this round, great problemsets with non-trivial test cases, reminds me of how airheaded i am

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

What is the counter for the following approach in D. Find the time intervals for each train where it will be skipped because of passenger trams. Then do something similar to prefix sums and find the time where minimum trains will be skipped.

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    4 years ago, # ^ |
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    The intervals follows a certain sequence for each time so they can be calculated.

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

Not to offend anyone, but keep in mind that the mere fact that tasks for 5-8 grade students are hard doesn't imply they're really good at CP. Measure the skills of participants by the number of problems solved, not by the hardness of the problems prepared by organizers.

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

me:

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

My video Solution For Problem A And Problem B. Hope you like it

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

Such a difficult div 2!

Do the students have a competition of 2 hours, as long as we do? That's terrible.

Besides, how old are students of 5-8 grades in Russia? They are so awesome to solve these difficult problems!

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

8776-4593= 4183 participants(including me) can't solve a single problem! :((

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

    There were 20,000 registrations, so about 16,000 people (including me).

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      4 years ago, # ^ |
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      Oh, you are right, I not include participants who make no submission :(

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      4 years ago, # ^ |
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      Note that there are a lot of people not submitting at all if they do not like the problems. Even if they are able to solve some.

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

Div2 spirit revived!!! Kudos to problem-setters

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4 years ago, # |
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My First Div 1 contest done !!

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

30 minutes more for such implementation heavy round would have helped :( nice problem set tho

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4 years ago, # |
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I think it was a hardest A problem in Div.2 among the things I saw

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

Can anyone share their code for C. and explain if possible..

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

    Solution C:

    Every type of flower has two values of happiness A and B, the main hint was that B value of happiness will only be used from a single type or we can say that value of B if needed will be taken from a single type of flower. Now we will loop for all m types and for every type we would check no of happiness of A-type which is greater than current B. So to take a B from a particular type of flower we would first use all the A-type happiness which are more than current B and for remaining(if any) we would use current A + (B * rem)-happiness. And looping over all flowers we can find the max ans.

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4 years ago, # |
  Vote: I like it +138 Vote: I do not like it

Codeforces Be Like: 48qmar

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

    Maybe there are many "wrong answer on pretest 2" and more submissions XD

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4 years ago, # |
  Vote: I like it +476 Vote: I do not like it

"This Sunday will take place All-Russian olympiad for students of 5-8 grades"

5-8 grade students :

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

    I wish i could upvote it more than once. xDD

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

    Well I am a student of grade 7 in China and I solve 3 problems in this contest.I think the problem is not too hard and it's suit for grade 5-7 if the contest get longer.The problems have some details but it does'n need too much algorithm knowledge XD

    All in all it's still a div 1.9 contest lol

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

      orz. When I was in your age, I was playing mud.

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4 years ago, # |
← Rev. 2   Vote: I like it +31 Vote: I do not like it

Next time I see a Russian 5th grader, I'm running away.

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4 years ago, # |
  Vote: I like it +18 Vote: I do not like it

I'm in 7th semester. Thinking of taking admission in grade 4. :-(

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

i got my mistake we need to count substring after doing changes not before 1 13 abacab?bacaba

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

I guess you did not have to be 2100+ to be in a Div1 contest! :-P

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4 years ago, # |
  Vote: I like it +28 Vote: I do not like it

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

Fastest system testing ever xD

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4 years ago, # |
← Rev. 2   Vote: I like it +44 Vote: I do not like it

I was about to say contest was tough but then I remember,

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

After how much time will the problems be uploaded in problemset?

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

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4 years ago, # |
← Rev. 3   Vote: I like it -59 Vote: I do not like it

Hello low rating

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

First time (after the initial struggling days) when I couldn't solve a single problem. Also, what do they teach 5-8 grade students in Russia? I'm very curious....

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4 years ago, # |
← Rev. 2   Vote: I like it -40 Vote: I do not like it

Loved the round. No speedforces, queueforces or mathforces. Strong pretests and interesting A (which is rare :p). Thankyou so much for this round.

PS : Missed getting to master due to WA's. Let's hope next round marks the day I become orange.

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    4 years ago, # ^ |
      Vote: I like it +28 Vote: I do not like it

    what was interesting in A? It was only implementation.

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

    Did you mean:

    Bad round. No logic. Just implement like a brainless robot and handle all corner cases. That is basically A,B in today's round.

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

      This was a just a normal round with a bit heavier implementation. A and B's are never that special so I don't really get why are you upset.

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

        I am not upset. But that round is not supposed to be for Div 2 xD. It is something similar to Div 1.5

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

      Just curious what are the corner cases for A? I did a fairly brute force implementation and there's not a single corner case. Same goes for B.

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

Weak pretest for B.

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4 years ago, # |
← Rev. 2   Vote: I like it +29 Vote: I do not like it

People after doing 1 question in this contest:

"pRobLeMsEt waS gReAt"

"tHaNks tO aUthERs"

"rAtInG dOEs nOt mAtTer"

"i aM gOnNA bE lEgUnDAirY gRaNDmAsTeR"

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

what we learned from this round: don't participate in rounds based on Olympiad :(

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

These Russians man! Flashbacks of Irodov failures was back today . Couldn't solve a single problem :')

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

I think Codeforces was making up for some previous easy div2 rounds.

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

    Or it just wants the community to see the power of Russian 5-8 graders.

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

This contest is too damn hard for Div.2, and the problem setter is shaco.

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

this contest suggests the best way to reduce the load on cf servers.

only 8k participants attempted the first problem out of 17k.

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

    and almost 3700 got it submitted correctly amazing stuff.

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

If you get WA on pretest 3 of Problem D, you can compare the submissions 87347894 and 87328303 to get the idea. :(

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

Codeforces Round #657 (Div. 2, based on All-Russian olympiad in the name of Keldysh)

Codeforces Round #657 (Div. 1, based on All-Russian olympiad in the name of tourist)

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

Is this contest really made for students of grade 5-8 in Russia ??? I am a final year UG in India and yet unable to solve even 2 problems on time. If those children solved more than that then "Unhe 21 topo ki salami"!!!

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

The Russian education system is praiseworthy truly! kids of 5-8th grade are solving these! Here in Bangladesh the education system must be changed like them too.

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4 years ago, # |
← Rev. 2   Vote: I like it +11 Vote: I do not like it

It's just me or someone also felt that problem A does not seem like div-2 A... it's more like div-2 C.

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

Can someone tell whether my logic for C is correct.

First chose all flowers of one type (let's call it good). I considered the good flower as the one with maximum value of a+(n-1)*b.
Maintain a priority queue containing other candidate flowers ( with values 'a' if it is used for the first time and 'b' otherwise).

I will iterate while I can increase my answer by dec the count of good flowers by 1 and taking the top element from priority queue. I am getting wrong answer on test case 3. My submission 87350479

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

    try this case, 

    3 2
0 7
5 6
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      4 years ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      By mistake I gave the wrong submission link (Now updated). The code for the above logic is giving 17 as answer which is correct I guess.

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

        Your assumption to start from the best possible case from beginning is not correct imo. Try this case, 

        4 4
8 1
9 1
2 4
0 5

        Also, according to me, you should be able to distinguish between the ai value and the bi value in the priority queue, because they hold different properties.

        You can refer the editorial for a better explanation.

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4 years ago, # |
  Vote: I like it +55 Vote: I do not like it

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

when will the rating update?

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4 years ago, # |
← Rev. 11   Vote: I like it 0 Vote: I do not like it

help！I don't know why it went wrong. Please T_T 87353258 here's my submission

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4 years ago, # |
  Vote: I like it -14 Vote: I do not like it

Please check the question difficulty before uploading, the div2 problems felt like div1 and clearly were way above the difficulty level. Disappointed

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

    If it is a mirror of the official event nothing really can be done about the difficulty

    One change I would've done though is explicitly specify in the announcement that difficulty would be higher than usual

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

I can't solved anything during 2 hours...

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

Div 1.5

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4 years ago, # |
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Felt like Div. 1.5 Perhaps.

Anyways, Great Contest. Absolutely Loved the Problems!

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

Guys, it may be a dumb question but I am forced to ask this after the setback in this round. Generally, A and B questions with n<=200000 and having a time limit of 1 sec have O(1) solutions. But here, in ques B r-l<=499999 but it still has O(n) solution, is TLE not expected in this case? Please solve my issue> Thanks in advance.

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

    It is because the number of test cases are less(upto 20), so total = 20 * 500000 = 10^7. So we do not need an O(1) solution here. As a generic rule you can follow this, if T*N <= 10000000, then O(n) solution usually passes.

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

Can someone help me understand where my code for problem C is giving wrong answer.Thanks in advance https://mirror.codeforces.com/contest/1379/submission/87357034

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

The hardest div2 I've ever solve

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

Older guys, please tell me if Mike Myrzayanov care about mentions in comments

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

I think Div2 B has weak test cases, my solution got AC https://mirror.codeforces.com/contest/1379/submission/87373765, but it should be TLE at this simple test case:

1
1 4 10000000000
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4 years ago, # |
  Vote: I like it +19 Vote: I do not like it

A small fact: there was no problem A in the Keldysh Olympiad, it was added to Сodeforces as a consolation.

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

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

I think the weak examples make the problem harder than other round...

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