Monogon's blog

Codeforces Round #658
By Monogon, history, 4 years ago, In English

Hello, Codeforces!

I'm very glad to invite you to Codeforces Round 658 (Div. 1) and Codeforces Round 658 (Div. 2). This contest will take place on 21.07.2020 17:35 (Московское время). In both divisions, you will have 2 hours to solve 5 problems (and one subtask). The score distribution will be announced closer to the start of the round.

Huge thanks to:

I've worked hard to ensure the pretests are short and the statements are strong. Remember to only read the problems that you can solve, and may you have the best of luck!

Because I know you all have so many questions, I have compiled an FAQueue

  • Q: Is it rated?
  • A: Yes

UPD Here is the score distribution:

Div. 2: 500 — 1250 — (1000 + 1000) — 2250 — 3000

Div. 1: (500 + 500) — 1500 — 2000 — 2500 — 3000

UPD Editorial

UPD I filled in the answer to the FAQ. Also, congrats to the winners!

Div. 2:

  1. badger_champion

  2. Mai_madarchod_hu

  3. rulai

  4. Vimmer

  5. niynip

Div. 1:

  1. Benq

  2. Um_nik

  3. KAN

  4. Petr

  5. ksun48

  • Vote: I like it
  • +2085
  • Vote: I do not like it

Comments Comments (272)
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4 years ago, # |
  Vote: I like it +660 Vote: I do not like it

As a tester, give Monogon contribution.

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4 years ago, # |
  Vote: I like it +493 Vote: I do not like it

As a tester, contest is bullshit.

Reference: https://codeforces.com/blog/entry/76777?#comment-613713

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

As a tester, I recommend reading FAQueue

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

So many questions and so few answers...

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

(and one subtask)

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4 years ago, # |
  Vote: I like it -69 Vote: I do not like it

Hope we don't encounter statements like Unrated and ruined, Upset and demotivated from mike.

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4 years ago, # |
← Rev. 2   Vote: I like it +172 Vote: I do not like it

Please use original FAQueue . Don't change.

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

    So this dude's pc is filled with screenshots I guess

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      4 years ago, # ^ |
      ← Rev. 3   Vote: I like it +28 Vote: I do not like it

      "A picture is worth a thousand words"

      Here we go again :)
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        4 years ago, # ^ |
          Vote: I like it +73 Vote: I do not like it

        Is this picture even relevant as reply to my statement or you just used it to prove that we are too dumb to understand xD

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

        Its like Div2 F for me, can you please explain this image

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

        "Ending racism by going vegan"

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          4 years ago, # ^ |
          ← Rev. 3   Vote: I like it +5 Vote: I do not like it

          False.

          Spoiler
          A picture is worth a thousand words
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4 years ago, # |
  Vote: I like it -53 Vote: I do not like it

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4 years ago, # |
  Vote: I like it +304 Vote: I do not like it

When you realize that Pretests are short and Statements are strong. ;-;
![ ](dd0)

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4 years ago, # |
← Rev. 2   Vote: I like it -63 Vote: I do not like it

Gonna be an amazing contest.

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4 years ago, # |
  Vote: I like it +52 Vote: I do not like it

Remember to only read the problems that you can solve New thing in the announcement

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

Hope to get some relief after the deadly Div2 round #657.

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

Meanwhile

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4 years ago, # |
  Vote: I like it +118 Vote: I do not like it

As a participant, I can ensure that good coders will have increased rating at my rating's expense. ;)

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4 years ago, # |
  Vote: I like it +104 Vote: I do not like it

did I read it right?

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4 years ago, # |
← Rev. 2   Vote: I like it +110 Vote: I do not like it

Why not this? (it's more accurate tbh)

Huge thanks to DeadlyCritic for being useless, and doing nothing, and not helping with problem in any kind.

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    4 years ago, # ^ |
      Vote: I like it +281 Vote: I do not like it

    Don't be so hard on yourself. It would be difficult to write good test cases without someone to fail all the problems.

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

      I ruined your unrated round, now it has non-zero chance to be rated, I'm so sorry about it. =(

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        4 years ago, # ^ |
          Vote: I like it -27 Vote: I do not like it

        Don't reveal that contest is unrated, only testers should know that (for now)

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

    ????????????????????????????????????

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

Where's the Monogon Platform?

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4 years ago, # |
  Vote: I like it +77 Vote: I do not like it

As a tester, this contest is unrated (for me).

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4 years ago, # |
  Vote: I like it -77 Vote: I do not like it

As an astrologist, I predict long queues again. XD

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

getting a hunch that this one is going to be one hell of a ride or maybe 567 is still haunting me who knows

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

    I think the previous round got into your head just too much that you can't even remember the Round number correctly!!
    #Round 657

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

Older guys, please tell me if Mike Mirzayanov cares about mentions in comments

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

"Remember to only read the problems that you can solve"

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4 years ago, # |
  Vote: I like it +282 Vote: I do not like it

Isn't it true Monogon ?

I-Do-One-Push-Up-Meme-Template

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    4 years ago, # ^ |
      Vote: I like it +119 Vote: I do not like it

    You know it’s true because his face matches Monogon’s profile picture

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

Such a great FAQ!!!

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4 years ago, # |
  Vote: I like it +151 Vote: I do not like it

zeoob.com_6boo0qgal9_photo.png

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

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

    Short pretests(x)

    Short but strong pretest with multi tests

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

Finally, Monogon is back!

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

Hello, I got quetion, in the problem D the test case include el case asdfghjk and the answer is 7, but the answer but the answer shouldn't be 6?. The string of the answer would be ddddgfee

sorry, ignore me, I was wrong about competition blog

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

Codeforces is very good!There are three to four competitions a week!

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

what does a short pretest mean? Less tests or just easier pretests? edit: sorry if im misinterpreting something or if theres some nuance but it would be really helpful if someone could answer instead of just downvoting

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

    "short pretests and strong statements" is a joke, it should have been "strong pretests and short statements"

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

      ah ok no wonder i couldn't understand any of the memes. Thanks!

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

I've worked hard to ensure the statements are strong

Don't tell me it's 2 hours of thinking for 4 lines of code..

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

all they are attempting to do is demotivate contestants and cause fewer people to participate

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4 years ago, # |
← Rev. 2   Vote: I like it -26 Vote: I do not like it

[Deleted]

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4 years ago, # |
  Vote: I like it -67 Vote: I do not like it

I'm very sorry to say but this round is going to be unrated

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

This month is already a disaster(in terms of our heartbreak during contests). I hope this contest will conduct smoothly. All I want to not see that popup message from mike! Let's hope for the best

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

Wow, Monogon is this you?

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

    Thanks. I can't believe you made me waste my time watching yoyo videos on Youtube

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      4 years ago, # ^ |
      ← Rev. 3   Vote: I like it +24 Vote: I do not like it

      communicating in internet is tough

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

        Bruh!

        It was about me watching anything. not blaming you. I found yoyo weirdly satisfying and ended up watching for like 30-60 mins xD (some 3A, 4A finals and stuff). It was a joke and wasn't blaming you, was instead thanking you lol.

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

          Ok, I got it now, sometimes it's hard to figure out the tone of the voice in a internet chatbox :v

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

            As someone rightly said :-)

            "communicating in internet is tough"

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

    I found another one, by Eva. I'm not 100% sure if both they are same, although they look same and have same name.

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

Great contest

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

What is mean by one subtask?

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

    One problem have easy and hard version *

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

constructiveforces!!!

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

Hope to become a master after this round.

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

Reading the blog, this looks to me a very different contest than usual. Really excited for this one!

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

Monogon how many shared problems will be?

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    4 years ago, # ^ |
      Vote: I like it +92 Vote: I do not like it

    div2(C+x) = div1(A+x), so 3 shared problems.

    The subtask is on div2C = div1A, so it is also shared.

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

    -1

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4 years ago, # |
← Rev. 2   Vote: I like it -48 Vote: I do not like it

do you guys know when will they announce the answer of this (Q Is it rated?). If announced can you leave the link below? If it's not rated I would rather virtual this round because in my time zone it's late night. Thanks!

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    4 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it 
    Answer will be available after the round .
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      4 years ago, # ^ |
        Vote: I like it -34 Vote: I do not like it

      is there a link that announced it will be available after the round?

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

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

What does it mean by (1000+1000) in score distribution of div2?

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    4 years ago, # ^ |
    ← Rev. 2   Vote: I like it +16 Vote: I do not like it

    Subtasks: C1 & C2 are worth 1000 each, so C is worth 2000 overall.

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    4 years ago, # ^ |
      Vote: I like it +40 Vote: I do not like it

    div2C has an easy version and a hard version. Each version is worth 1000 points, so 2000 total.

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

I'm new to cf, but why do they tell you do only read problems you can solve?

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    4 years ago, # ^ |
      Vote: I like it +62 Vote: I do not like it

    Because if you know you can't solve them, why waste the website's precious resources by requesting for the problem page?

    It's codeforces new strategy to reduce server load.

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

    They meant "only solve the problems you can read".

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

After seeing score distribution

Good luck with question B.

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

I'm not strong enough to compete with all of you >_<

I need do more problems, luck for you!

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

    Is that #657 that got you demotivated?

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4 years ago, # |
  Vote: I like it +90 Vote: I do not like it

In div2, D is worth 12% more than C (that's 2250 vs 2000 points). In div1, those two problems differ by 50% (1000 vs 1500 points). Maybe these two fractions shouldn't be equal but should they differ that much? I don't think so.

#geometricprogressionmatters

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

    well in div2 people usually solve problems in order a b c d etc . and it takes time to solve c1 and c2 and that time is gonna be counted for both of the problems . (penalty would be too much for c1+c2) i think its the reason .

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

      You ignored the fact that penalty is in proportion with the problem points.

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

awaiting a good round

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4 years ago, # |
  Vote: I like it -27 Vote: I do not like it

Hope,It will be a great round and we all get high rating, in shaa allah.

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

so your compiler of FAQ didn't show any output !

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

System test would be a disaster today. Why?

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

For no reason I hate subtasks. :/

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

Remember : Only read the problems that you can solve. If you can't solve it, don't read it in the first place.

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

    Interestingly, all 4 of these are helpful advice:

    • Remember to only read the problems that you can solve (waste of time)
    • Remember to only read the problems that you can read (waste of time)
    • Remember to only solve the problems that you can solve (waste of time)
    • Remember to only solve the problems that you can read (you won't solve it anyway)
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4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Problem page for me keeps loading.. It's showing nothing

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

Problems are great, I wish the queues don't get really long this time.

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4 years ago, # |
← Rev. 3   Vote: I like it -9 Vote: I do not like it

I had a correct answer to C1 and I resubmitted by mistake because I had both C1 and C2 open and I was trying to submit to C2 the improved solution. This cost me 200 points :(((

( I submitted C1 after 30 minutes and the second one was after an hour)

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

    Same with me. Isn't only the first accepted submission supposed to count? Or have the rules changed? :(

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

      But pretests passed does not mean it will always pass system testing. The initial solution may not be totally correct so one might want to resubmit a better solution that also passes pretests, but would also pass system testing.

      In addition, haven't the rules always meant that resubmission costs 50 points no matter what?

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

        Well it would be nice if you could choose (the default is the last one, but you can choose a previous revision).

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

    In D won't the answer of last test case be YES?

    If we take A-[4 3 2 5 1 11] and B-[9 12 8 6 10 7] .

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

      No, because 11 and 9 have to be in the same array. The big observation is that if there is a maximum up to now (which 11 is), then until you pass that max, all of the following elements must be in the same array as the max. That's because if they are smaller, and they were in the other array, they would have been picked before the max.

      Think about your example. after 4 3 2 5 1 where chosen, there could be two scnarios:

      a = 11 ...

      b = 9 ...

      or:

      a = 11 9 ...

      b = ....

      The first case CAN'T happen, because 11 was picked, so 9 has to be in the same array with 11.

      My solution simply groups together elements that have to be in the same array, and then I create an array of the number of elements that have to be grouped together. in the last test case, the list of elements is [{4,3,2}, {5,1}, {11,9}, {12,8,6,10,7}] which turns into [3,2,2,5] Now each of these groups can be assigned to any array, so you need to find a subset of elements who's sum is equal to n, which is the subset sum problem. Then I copied this dp solution: https://www.geeksforgeeks.org/subset-sum-problem-dp-25/

      And all you have to do is use this function and return "Yes" if True or else "NO". My solution:

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

Hmmpfffh! Totally disappointed by the explanation of Problem C. It's been 20 minutes and still not able to understand it fully.

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

    What part of it do you not understand? In one operation you take some prefix, flip the bits and reverse it. Then you want to turn string a into string b in 3n operations (or 2n, for the second subtask).

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

      It wasn't clearly mentioned that we'd to reverse and then flip the bits (doing vice versa would yield different results!). Overall, it was a confusing statement but a nice question.

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        4 years ago, # ^ |
        ← Rev. 3   Vote: I like it +5 Vote: I do not like it

        Can you give an example of a string such that reversing the bits and then flipping them produces a different string than flipping and then reversing them?

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

I have submitted code B twice. Then I realize the first code was not wrong. Is there any way to cancel the last submission? My rating has changed a lot because of the last submission.

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

    No. There is no way to cancel any submission.

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

    i think you can lock one of your submission for judging.
    on submission tab there is lock icon.

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

      Are you sure that it can help?

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

    It won't change A LOT.

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

Nice problemset! Really enjoyed the contest! :)

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

    In D won't the answer of last test case be YES?

    If we take A-[4 3 2 5 1 11] and B-[9 12 8 6 10 7] .

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

      It would give 4 3 2 5 1 9 11 12 8 6 10 7. Which is different from the last test case. Also, don't leave your comment under other's comment.

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

      merge(a,b)= [4,3,2,5,1,9,11,12,8,6,10,7] not [...11,9...]

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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 B. Hope you Like It.

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

Am I the only one who solved Div1A2 using Treap?

Solution: smash me

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

😩😩😩😩
please make contests harder, this one wasn't fun

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

    previous one was pretty hard :p

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

    Speaking for div1, lots of contestants only manage to solve A or A+B, and the competition boils down to solving these 2 faster (=typingforces) — and the scoring distribution among the second half of participants doesn't reflect their skills. Harder contests will make this even more problematic.

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

      oh i forgot to look at the standings and realize C wasn't easy

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

        Out of curiosity, is C the type of problem you have been advocating for? It seemed to be a pretty good example of easy solution, nontrivial implementation (assuming my solution is correct).

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

          implementation was trivial, sorry

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            4 years ago, # ^ |
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            :’( I should learn how to code

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            4 years ago, # ^ |
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            Either we're talking about different problems or we have a different notion of what trivial is

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    4 years ago, # ^ |
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    how did you manage to get WA on A1 and AC on A2?

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      4 years ago, # ^ |
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      I submitted different codes

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        4 years ago, # ^ |
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        oh yeah, I saw they were slightly different, so wondering what the key difference was

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          4 years ago, # ^ |
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          i don't know, check yourslef

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            4 years ago, # ^ |
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            Hmm.. now I think those two smiley faces in your profile actually symbolise two people on one profile.

            Adam and Eva

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4 years ago, # |
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How to solve C2? Was it based on a similar technique as C1 with optimization, or an entirely new way?

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    4 years ago, # ^ |
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    Instead of reversing string i access string form back and front(using deque). Everytime making a character equal i remove that from a and b. And i handle flip of bit using equal and not equal.

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    4 years ago, # ^ |
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    Use two pointers

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4 years ago, # |
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Please, someone, help me with C2. Thanks in advance

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    4 years ago, # ^ |
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    make a either all 0 or all 1 than solve it from the last index

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      4 years ago, # ^ |
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      Yaa that I get but what about reversing string in each step??

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        4 years ago, # ^ |
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        there will be no need to reverse, think for a moment u will get the idea

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      4 years ago, # ^ |
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      this is much better what I was doing was adjusting the first index of a according to the b[i] and then reversing. before checking the value of a[i] I was just adjusting its value. I saw the patter to how I should adjust value just before 2 minutes at the end :(

      I missed div2 C2 by 10 sec. I am pissed i am always careless.

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      4 years ago, # ^ |
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      how do you make everything 0 or 1?

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        4 years ago, # ^ |
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        let say we r at index i from i-i-1 a is either 0 or 1 so if i is same then we have to nothing otherwise change 0-i-1 values

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    4 years ago, # ^ |
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    i did it using two pointers and flag that shows the fact that s[l,r] has been reversed

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4 years ago, # |
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Any hints for D?

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4 years ago, # |
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Is the required and sufficient place where a snake can flip around a vertex $$$v$$$ connected to 3 disjoint paths with length >= snake length? I assumed so and looked for all such vertices $$$v$$$ which the snake can reach with its head/tail, but got WA, so I'm wondering if I have a bug or the idea is wrong.

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4 years ago, # |
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now after problem C1, I won't forget that "STRING IS IMMUTABLE"

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4 years ago, # |
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D1D was inspired by task from MIPT workshow (2019 day 5 B Little Worm). Both are extremely shit.

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    4 years ago, # ^ |
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    ... which is taken from Polish ICPC stage (AMMPPZ in Cracow). I was one of the organizers of that contest and I was too lazy today to search for the solution code — solving other problems seemed more fun.

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      4 years ago, # ^ |
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      I've wasted a lot of time trying to use these codes — I think that it's hard to use them as the problem was slightly different.

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        4 years ago, # ^ |
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        I've used the code for the Little Worm problem to check if we can reach a diameter. It's a bit easier to solve it if our snake is already located on a diameter.

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      4 years ago, # ^ |
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      Well, code from that problem didn't help me much, although some parts of the solution share the same idea.

      Also, I liked both problems and I don't think they are too similar: in older problem, the start and finish positions were disjoint as paths, in today's problem they coincide. I hope I will not see the problem without position restrictions in a contest, though :)

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4 years ago, # |
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How to solve C1 with 3*n operations?

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    4 years ago, # ^ |
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    change elements of a from the end one by one

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    4 years ago, # ^ |
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    solve from the last index if it is correct than just remove it from a if it is not make it correct with the help of 1 index

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    4 years ago, # ^ |
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    I got pretests passed both subtasks with a simple greedy algorithm:

    Spoiler
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    4 years ago, # ^ |
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    You can always use the zeroth index character for swapping. Iterate from back of first string if its different , then check for conditions.In case — the first character of first string is different from character of second string , you can just perform operation once and update the string : else you need to perform the operation two times first on the very first character of first string which makes it different from character you wanna change for in second string and then perform operation on prefix of first string of desired length.

    Ps: sorry for bad english!

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4 years ago, # |
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need a quick editorial now

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

Div2A, Div1B, Div1C, Div1D — Codeforces? More like YesOrNoForces

(I'm actually mocking people that complain about the lack of diversity in problemset by checking similarities that are not relevant).

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4 years ago, # |
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C2 is very interesting. Do we need any data structure to proceed operations?

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    4 years ago, # ^ |
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    keep a variable for zeroth index character and one variable for number of reverses.Iterate from back and keep updating both of the variables as required.

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    4 years ago, # ^ |
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    No, I solved it with array only. Link

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4 years ago, # |
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in C-2 has anyone thought of doubly LinkedList kind of thing with a status variable?

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    4 years ago, # ^ |
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    Yess. I think will enable us to solve using in 2n operations and O(N) time.

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    4 years ago, # ^ |
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    I used deque

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4 years ago, # |
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Do anyone know how to bitset in dynamic length?

Please help. I think I got TLE on pretest 3 in C2 coz of inefficient implementation of this :/

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    4 years ago, # ^ |
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    Same happened with me.

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    4 years ago, # ^ |
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    No. I think the main issue is that your solutions is $$$O(n^2)$$$

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      4 years ago, # ^ |
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      Yes My code was doing in 2*n operations but time is O(n^2).

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        4 years ago, # ^ |
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        So bitset probably won't help in this case

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          4 years ago, # ^ |
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          So How to resolve it or reduce the complexity?

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            4 years ago, # ^ |
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            Editorial has already been released

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    4 years ago, # ^ |
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    You can just keep two pointers. Its pretty fun to code it that way.

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4 years ago, # |
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Anybody else wondering why problem B have more points than C1 ?

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4 years ago, # |
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Is it rated? :)

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4 years ago, # |
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The sample test case explanation was enough for solving Div2-B.. but I didn't care to read it at first because I thought NIM was always based on XOR.. nvm got AC after reading it.

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    4 years ago, # ^ |
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    whats your approach and what you conclude from test case?

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      4 years ago, # ^ |
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      "The second player should take the stones from the second pile because the first pile is empty. He will take 1 stone because he can't take any other number of stones."

      The first player can always force ze second player to take out any desired no of stones(which shall benefit him) in every turn , unless he is forced in the same way if there's only 1 stone to take out in the start.

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4 years ago, # |
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As expected nice problemset. Next time i will like to solve problem named Monogon and rescheduling.

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4 years ago, # |
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In D won't the answer of last test case be YES?

If we take A-[4 3 2 5 1 11] and B-[9 12 8 6 10 7] .

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    4 years ago, # ^ |
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    No, if you merge these 2, you will get

    [4, 3, 2, 5, 1, 9, 11, 12, 8, 6, 10, 7] which clearly not equal.

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4 years ago, # |
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Last div2 was like div1 and this div2 was like div3. :-)

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    4 years ago, # ^ |
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    -1+1=0. "Perfectly balanced as all things should be"

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      4 years ago, # ^ |
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      Some things can't be assume to be balanced like "blood pressure" if for half of the time it is above than normal and other half of time it is below normal then you can't call a patient as normal. I think same goes for difficulties in contest.

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4 years ago, # |
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Video Solution to C1 and C2.
I have discussed how to solve in 2n operations and N^2 time, how to solve in 3n operations and O(N) time.
Finally optimized 2n operations and N^2 time to 2n operations and O(N) time.
Enjoy watching: https://youtu.be/TSr0x3EBWSg

I will most likely make for D also :)

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4 years ago, # |
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Am I only the who who get WA in problem A on test 5(System tests). It's strange. UPD 87526239

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4 years ago, # |
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It's good to see a geometry problem from time to time, even if it's nothing new and groundbreaking :D

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    4 years ago, # ^ |
      Vote: I like it +54 Vote: I do not like it

    It's good to see Errichto in comments from time to time, even if it's.. wait what.. no..

    Instead I want to see you on contests page hosting one. :D

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    4 years ago, # ^ |
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    It's good to see you in comments from time to time even if it's just for the purpose of gathering some upvotes and increasing your overall contribution through random rheoteric comments.

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    4 years ago, # ^ |
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    It's good to see you solving geometry problems so easily, even when most of the people find them really hard.

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4 years ago, # |
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I think the time limit of Div.2 D is too tight. I got the right solution when it was only 30 second before finish, but get TLE disappointedly. My TLE Code

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4 years ago, # |
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From many past contest, counting this one I am having a syndrome that I call "Coded Bug Free Solution after 10 mins of contest is over."
Any helpful tips to get the job done without bugs within time limit??

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

    Every time you find a bug, answer 2 questions:

    • Why did this bug appear
    • How do I change my coding practices so that a bug like that will never appear again.
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      4 years ago, # ^ |
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      That's a good idea. I think I should maintain a journal about it.
      Thank you for your advice!

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    4 years ago, # ^ |
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    I think instead of trying to code without bugs, learn how to debug.

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      4 years ago, # ^ |
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      What if I learn both to code without bug(Amortized analysis!) and also to debug.

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4 years ago, # |
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How to solve Div2E?

I used DP on state (i, j, s) which means merge(a[1:i], b[1:j]) (the variable s is used to determine the (i+j)-th element of p belong to a or b)

Let x be the nearest index such that x > i+j and p[x] > p[i+j].

Then the transition is to try to append the x-th element to a or b.

I'm not sure about my solution (although pretests passed). Does anyone have another idea? ^_^

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

I think I may finally hit purple thanks to your contest :) So thank you!

I had to think hard the entire time. I especially enjoyed Div2 D. Not sure if it was the intended solution, but I ended up doing a subset sum problem on sizes of segments in the array, after realizing that the array could be split up into segments in a natural way.

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

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4 years ago, # |
  Vote: I like it +143 Vote: I do not like it

Nice round imo. I think that for a long time there was no geometry and it's generally nice when problems touch various topics and ad-hocs aren't in majority. Kudos

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4 years ago, # |
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(I already posted it but it was deep in a thread so probably people won't notice it)

An elegant solution that probably works for problem D (finished it 10 seconds after contest was over so not sure :| ): EDIT: Well, it TLEs on test case 30, probably just because of python :|).

Spoiler
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4 years ago, # |
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Interesting observation: when you get WA on pretest 1 on problem C (div1), if you click on your submission, checker tells you what's wrong with your output and provides some additional information (in this case, for example, that in the test case 2, x is not what it should be). You also don't get any penalty for WA1.

This is kinda non-obvious, and gives some slight advantage to people who decide to check the details of the submission. Maybe we shouldn't print stuff like that in a checker?

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    4 years ago, # ^ |
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    Printing helpful information in the checker has always been preferred. But I believe it's not available during the contest, and you only see the verdict itself.

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      4 years ago, # ^ |
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      No, I actually saw the exact same thing during the contest too, and used it to debug my solution in the last minutes. In was definitely easier than understanding what's wrong myself.

      I agree that printing stuff in the checker is helpful, maybe CF shouldn't show checker output to the participants then.

      Anyways, thanks for the round, I liked the problems! (even though I wrote a silly N^2logN solution with N Fenwick trees on B :)

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        4 years ago, # ^ |
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        Why not show the output? This is the sample, so participants could very easily run their program against the case and figure out what is wrong (except if maybe they misunderstood some part of the problem, in which case it is very much in the spirit of samples to give some help as to where the issue is).

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          4 years ago, # ^ |
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          Well, you never really know what checker will say (just "answer is not correct", or "answer is not correct because of X"). To find out you need to get WA1 and check out the submission page, which is not obvious, and may give advantage to people who happen to do so.

          And output in the style of "answer is not correct because of X" can be very helpful when you are constructing the answer, and using sample 1 to see if your construction is correct.

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      4 years ago, # ^ |
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      It gives additional information on contest time too. Today i got it on D2C1.

      And i have seen this few more times before. Sometimes it helps to verify if i understand problem in wrong way.

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

Very bad pretests in A2. Why didn't you add the max pretest? I thought that if the solution passes pretests in terms of perf, it would be accepted.

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

Mai_madarchod_hu is in 2nd position of today's round. I am just imagining the post with the winner's list XD

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4 years ago, # |
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I have a rating of 1282(Pupil). Just wanted to ask if Round 658Div2 will be rated for me and will it affect my graph, as it is not updated as of now. I'm new here, please let me know. Thanks

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4 years ago, # |
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I wish the contests would have been 2:15. Then I would have got D

:((((

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4 years ago, # |
  Vote: I like it +51 Vote: I do not like it

Hello, Monogon

I think the main test data for div2D is weak.

Check this 87590140 for example.

Input:

1
9
2 1 4 3 7 6 5 10 9 8 13 12 11 18 17 16 15 14

The answer should be YES, but this submission provides NO. Please have a look into it.

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

    I'm sorry that the tests seem to be weak. The final standings are already out, but I can add test cases like this to the system tests so that more wrong submissions can fail in practice mode.

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      4 years ago, # ^ |
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      Monogon Please add this test case as well. It seems to give NO as the output for some codes (including the one mentioned above) whereas the answer should be YES.

      Input:
      1
      16
      27 1 28 2 29 3 4 5 6 7 30 8 9 10 11 12 13 31 14 15 16 17 18 19 32 20 21 22 23 24 25 26

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

Thanks for the round!

My screencast: https://youtu.be/VaCa0CQpYbo

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

Monogon, Thanks for the contest! I liked it :)

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4 years ago, # |
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This was my fifth contest and I liked it. I could have solved the unmerge problem. Ran out of time. Need to get quicker

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

The first time I solved a D and got Pretest passed, they failed system test and got TLE.....
Maybe the pretests were infact short :P

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

Maybe only me can't find the operations that transform the string a into b in more than 2n steps...I think 1A1(2C1) is a little strange.Because if we fix the bits one-by-one,it's so easy to find a 2n steps in O(n) time.But the contest is nice and thank for +77 lol

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    4 years ago, # ^ |
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    There were some random solutions people used, several 3*n solutions worst case, and a couple 2*n solutions that weren't optimized to n^2 in testing. Monogon posted some of them in the editorial if you're curious.

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

qwq.png

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4 years ago, # |
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Does anyone have any idea behind the reason for rolling back the ratings?
Did I miss something?

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

    It usually happens when some cheaters are caught, causing the rankings to be re-judged.

    I think the same must have happened this time too.

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      4 years ago, # ^ |
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      Could this be the reason?

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        4 years ago, # ^ |
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        Yeah, but Monogon clearly states that he will amend test cases for the practice mode, I don't think that it would affect the contest's ratings rollback

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

In D2C2, I tried implementing the solution 1 of editorial and it works fine on my system. However, it is giving Runtime Error on submitting. I've tried debugging but to no avail. Can someone help find the mistake(s)? My solution: https://codeforces.com/contest/1382/submission/87742162

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

Mai_madarchod_hu orz

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