Markadiusz's blog

By Markadiusz, 6 weeks ago, In English

Hello Codeforces!

Asymmetry and I are glad to invite you to Codeforces Round #743 (Div. 1) and Codeforces Round #743 (Div. 2), which will be held on Sep/18/2021 17:35 (Moscow time).

Each division will have 6 problems and 2 hours to solve them. All problems were written and prepared by Asymmetry and me. The round will be rated for both divisions.

We would like to thank:

We've put great efforts into preparing this round and we hope that you will enjoy it.

Good luck!

UPD: We would also like to thank Ari for testing the round and KAN for translating the statements into Russian.

UPD2: Here are the scoring distributions:

Div. 1: $$$500$$$ — $$$1250$$$ — $$$1750$$$ — $$$2500$$$ — $$$2500$$$ — $$$3250$$$

Div. 2: $$$500$$$ — $$$1000$$$ — $$$1500$$$ — $$$2250$$$ — $$$2750$$$ — $$$3500$$$

UPD3: We are sorry that the round became unrated due to the long queue. We hope that you enjoyed the problems anyway.

Winners

Congratulations to the winners!

Div1.

  1. tourist

  2. Benq

  3. peehs_moorhsum

  4. ecnerwala

  5. Molewus

Div2.

  1. ChenRui

  2. rainboy

  3. OguriCap

  4. _October

  5. Dallby

UPD4: Editorial is out!

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

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6 weeks ago, # |
  Vote: I like it +270 Vote: I do not like it

Since 1-gon had ascended into the coordinator realm, I became the new VIP tester.

btw, as a VIP tester, I tested

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6 weeks ago, # |
  Vote: I like it +179 Vote: I do not like it

As a tester, I wish you sunny weather ^^

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6 weeks ago, # |
  Vote: I like it +253 Vote: I do not like it

As coordinator I want to thank the VIP authors Asymmetry and Markadiusz

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6 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it
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6 weeks ago, # |
  Vote: I like it -38 Vote: I do not like it

When will next div 3 round happen??

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

Looking forward to participate

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6 weeks ago, # |
Rev. 3   Vote: I like it -122 Vote: I do not like it

Challenge:hit 200 downvotes under this comment (bye bye contribution)

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6 weeks ago, # |
  Vote: I like it +145 Vote: I do not like it

As a coauthor, I wish everyone best of luck and a positive skill delta after the contest.

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6 weeks ago, # |
  Vote: I like it +46 Vote: I do not like it

As a tester, I wish everyone happy rating :)

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6 weeks ago, # |
  Vote: I like it -21 Vote: I do not like it

Why the huge difference between C and D problems !!!! ** it was the first time i see such a difference ** Is it okay >>

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

Don't wanna miss the contest but suffering from anxiety and depression for the past few days.Hopefully I will overcome anxiety and depression soon and start playing in contests

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    6 weeks ago, # ^ |
      Vote: I like it +15 Vote: I do not like it

    Mental health is much more important than competitive programming, so take it easy, hoping for some well wishes from a fellow stranger.

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

      Yeah,I am going to meet my psychologist today for that reason.Being mentally strong is important in cp too.Because you can have all the skills in the world but if your mind is unstable during 2 hour contest I Don't think you can deliver your best.Anyway,I am going to consult my psychologist soon and come back strongly.And thanks for your kind words.Cf has really been like a family to me

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

        I hope you become well in the coming days and I wish the best of luck for you in the contest

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          5 weeks ago, # ^ |
          Rev. 3   Vote: I like it 0 Vote: I do not like it

          Thanks for your support.Unfortunately I am not stable enough to give today's contest but hopefully I will be giving every contest in cf from 23rd September.I wish you all the best in today's contest

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            5 weeks ago, # ^ |
              Vote: I like it 0 Vote: I do not like it

            Take good rest bro ,and come back strong

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              5 weeks ago, # ^ |
                Vote: I like it 0 Vote: I do not like it

              yeah sure buddy, pray for me.Good luck for today's contest.Wish you high rating!!!

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            5 weeks ago, # ^ |
              Vote: I like it 0 Vote: I do not like it

            Thanks turzo_sawroop , I'm still working on a strategy to reach a better level but I think it will take some effort and time to reach a higher rank

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              5 weeks ago, # ^ |
              Rev. 4   Vote: I like it 0 Vote: I do not like it
              • solve 1/2 1200 rated problems

              • solve 1/2 1300 rated problems

              • solve 1/2 1400 rated problems

              Do these three things everyday

              • solve 2 1500 rated problems

              Do this in every week

              • Don't miss any contest

              Do all these in the next 2 months and be confident and always believe you can be as good as anyone.

              Hopefully your rating will be between 1200-1300 after that

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                5 weeks ago, # ^ |
                Rev. 2   Vote: I like it +11 Vote: I do not like it

                From my experience, I think you just need learn the basic technique to get rating >1600. Do too much easy problem doesn't make your skill better.

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                  5 weeks ago, # ^ |
                    Vote: I like it -6 Vote: I do not like it

                  I agree but my learning was always extremely problem-solving oriented.Even I never studied binary search.I felt the importance to learn it only when I had to do some binary searching in a problem. Well, every human-being is different.So, strategies may vary

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                  5 weeks ago, # ^ |
                    Vote: I like it 0 Vote: I do not like it

                  I'm trying to do this recently and I'm learning new topics as well but it's not always easy to use new topics in solving problems it takes some time

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                5 weeks ago, # ^ |
                  Vote: I like it 0 Vote: I do not like it

                Thanks for the advice I'm going to try it and maybe I will try solving problems on other websites as well

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

kickstart round is also scheduled from 17:00 UTC on the same date, can u guyz plz prepone the round?

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6 weeks ago, # |
  Vote: I like it -89 Vote: I do not like it

As someone who wants to hit -69 contribution mark, I request y'all to downwote for high ratings. thank you!

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6 weeks ago, # |
  Vote: I like it +122 Vote: I do not like it

As an old, almost retired contestant who grew up in the same school as the authors, I'm thrilled to see what my younger comrades prepared :)

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6 weeks ago, # |
  Vote: I like it +8 Vote: I do not like it

Interesting division 2 scoring distribution. I expect (and hope for) a sizeable jump between C and D.

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

Early scoring distributions! :D

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

I am a newbie and started giving contests on this platform from this month itself. Excited for this another upcoming contest!! All the best everyone!

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

    I am a specialist and started giving contests on this platform from last year itself. Excited for this another upcoming contest!! All the best everyone!

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      5 weeks ago, # ^ |
        Vote: I like it +1 Vote: I do not like it

      I am a pupil and started giving contests on this platform from this year itself. Excited for this another upcoming contest!! All the best everyone!

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

        I am a future expert and started giving contests on this platform 20 yrs ago. I was excited for this then upcoming contest!! All the best everyone!

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5 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

Target to solve A, B & C asap to get a good rank, D seems to be much harder based on rating distribution.

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5 weeks ago, # |
  Vote: I like it -19 Vote: I do not like it

Noice looking contest good luck :> HEHEHE HA

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5 weeks ago, # |
  Vote: I like it -8 Vote: I do not like it

I hope a good result for everyone !

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5 weeks ago, # |
  Vote: I like it -9 Vote: I do not like it

Hope all get positive points :)

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5 weeks ago, # |
  Vote: I like it -39 Vote: I do not like it

many tks for your efforttt hope to up rate uhuhu

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5 weeks ago, # |
  Vote: I like it +14 Vote: I do not like it

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5 weeks ago, # |
  Vote: I like it -12 Vote: I do not like it

Very good stuff worked to prepare this contest I guess, thanks

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5 weeks ago, # |
  Vote: I like it -70 Vote: I do not like it

As your father,I want some contribution

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5 weeks ago, # |
  Vote: I like it +48 Vote: I do not like it

I just want to say that 743 is a prime number.

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5 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

Suiiiiiiiiiiiiiiiii!

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5 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

all the best to all

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5 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

Wish Me Cyan !!!

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

"An Unexpected error" is showing up whenever i am submitting my solution!!! Anyone else facing the same issue????

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5 weeks ago, # |
  Vote: I like it +17 Vote: I do not like it

Queue :(

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5 weeks ago, # |
  Vote: I like it +8 Vote: I do not like it

too long queue :(

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5 weeks ago, # |
  Vote: I like it +9 Vote: I do not like it

A contest after ages and what we see is a Queue

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5 weeks ago, # |
  Vote: I like it +84 Vote: I do not like it

unrated please

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5 weeks ago, # |
  Vote: I like it +12 Vote: I do not like it

QueueForces T_T.

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5 weeks ago, # |
  Vote: I like it +10 Vote: I do not like it

such a long queue , it has to be unrated .

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5 weeks ago, # |
  Vote: I like it +11 Vote: I do not like it

15 minutes since A is in queue.....

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5 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

Thinking for 10min shall I get TLE in problem B or not :)

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5 weeks ago, # |
  Vote: I like it +8 Vote: I do not like it

too long queue:(

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

while true: dequeue()

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5 weeks ago, # |
  Vote: I like it +8 Vote: I do not like it

please extend the contest by 15 mins at least due to the long queue.

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    5 weeks ago, # ^ |
      Vote: I like it -48 Vote: I do not like it

    don't make it unrated because the problems are really good.

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      5 weeks ago, # ^ |
        Vote: I like it +23 Vote: I do not like it

      just cause you solved , this is unfare to wait for 20 min to get wa verdict

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        5 weeks ago, # ^ |
          Vote: I like it 0 Vote: I do not like it

        i never said it is fair to not make it unrated but an alternative is to extend it since a contest is much awaited by everyone.

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

          imagine waiting for B and then getting wa after 30 min for some silly stuff , meanwhile some guy solved after 30 min and submitted in first go , I dont think extending contest will justify now

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      5 weeks ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      So good that you submitted A twice

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    5 weeks ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Its not possible google kickstart is on the way.

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      5 weeks ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      for me at least any CF contest >>>> any other contest except(ICPC).

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        5 weeks ago, # ^ |
          Vote: I like it 0 Vote: I do not like it

        Lol, I guess you are only thinking about you. Possibly everyone here will participate in kickstart.

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5 weeks ago, # |
  Vote: I like it +26 Vote: I do not like it

This should definitely be unrated!

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5 weeks ago, # |
  Vote: I like it +40 Vote: I do not like it

The contest must be unrated . Such a long queue

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5 weeks ago, # |
  Vote: I like it +29 Vote: I do not like it

I think there should be a clear guideline in Codeforces when the round should be unrated. Because queue in first few minutes can severely affect the rating in case there is a difficulty gap in the problems.

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

Why so long queue??

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5 weeks ago, # |
  Vote: I like it +1 Vote: I do not like it

Too long queue, please, make this round unrated

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5 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

The queue is too long today? i submitted a solution 5 mins back and still it's not judged yet.

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5 weeks ago, # |
  Vote: I like it +31 Vote: I do not like it
Queueforces ;-;
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5 weeks ago, # |
  Vote: I like it +8 Vote: I do not like it

MikeMirzayanov what type of queue is used for submissions, my submission at 7 min is not yet judged but some user's 12 min sol are judged. Please help!!!!

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5 weeks ago, # |
Rev. 2   Vote: I like it -7 Vote: I do not like it

If unrated, my possibility of becoming CM will be ruined for this time.

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5 weeks ago, # |
  Vote: I like it +41 Vote: I do not like it

Unrated please!!

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

Why queue is so long?

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

Is it unrated?

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5 weeks ago, # |
  Vote: I like it +12 Vote: I do not like it

20 mins just to get wrong answer so sad

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5 weeks ago, # |
  Vote: I like it +9 Vote: I do not like it

The contest should be unrated cause of long queue!

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5 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

@mike @contest admins please give the info whether it will be rated or not

so that we can do some other things.

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

I am waiting for more than 30 minutes, still my solution is not judged .

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5 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

Next contest shall be to program decent queue

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5 weeks ago, # |
  Vote: I like it +1 Vote: I do not like it

Disgusting queues

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5 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

To be honest round should be unrated as 30 minutes are too much just to get a wa.

Nothing to take from Authors you did a phenomenal job

but it should be unrated.

30 minutes are too much guys/

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5 weeks ago, # |
  Vote: I like it +1 Vote: I do not like it

Such a bad queue :(

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5 weeks ago, # |
Rev. 2   Vote: I like it +19 Vote: I do not like it

Me: submits

Codeforces: Click this

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    5 weeks ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    It is also gonna cost penalty and pain if it comes out to be wrong.

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5 weeks ago, # |
  Vote: I like it +8 Vote: I do not like it

Imagine submitting before 30 min and getting WA. :))

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5 weeks ago, # |
  Vote: I like it +17 Vote: I do not like it

It'll be unrated, there's no point participating. There's no better way to compensate for never getting results on pretests.

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

queueForces

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5 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

Finally it's unrated now !!

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5 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

it's unrated no doubt just have fun solving problems

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5 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

Good bye , tata, Allah Hafiz

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

Nice round. It's a pity that this round is unrated(

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5 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

F

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5 weeks ago, # |
  Vote: I like it +1 Vote: I do not like it

Great questions, sad the round will be unrated.

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5 weeks ago, # |
  Vote: I like it +18 Vote: I do not like it

Second person to solve C only for round to get unrated xD

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5 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

Best round but very sad that this became unrated

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5 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

I thought the problems were really nice, so I'm disappointed that the round went unrated.

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5 weeks ago, # |
  Vote: I like it +18 Vote: I do not like it

will it be unrated for div 1 also?

still 0 announcement

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    5 weeks ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    yeah! it was announced that it will be unratted for div1 too

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5 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

Bye Bye, See you at Google kickstart

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5 weeks ago, # |
  Vote: I like it +11 Vote: I do not like it

When I solved A and B under 15 minutes, the round gets unrated. Of course, why not :(

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5 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

Read first three problem . The questions were interesting . Sad that the round in unrated :(

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5 weeks ago, # |
  Vote: I like it -8 Vote: I do not like it


Thousands of submissions are in queue and only one of them is running. why?

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5 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

thanks for hosting I enjoyed the problems I attempted. sad the queue times were too long.

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

B and C were so beautiful, very sad it's unrated.

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5 weeks ago, # |
  Vote: I like it +1 Vote: I do not like it

this is not expected from such big organization please fix this for once I did so good in the contest (a and b) this fast (i'm newbie) :(((

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5 weeks ago, # |
  Vote: I like it +131 Vote: I do not like it

Results of other OJ:

Result of Codeforces:

lol

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5 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

It's a pity that the round was unrated. But, the tasks were interesting. Thanks!

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5 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

After so much time as a beginner i am finally able to solve a few and it is unrated now. Tbh i am pissed off :(

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5 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

I have a theory that most problem-setters make tough problems just to avoid queue issues. LOL! Nice contest, btw.

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5 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

problems were really good and don't deserve unrated(;_;)

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5 weeks ago, # |
  Vote: I like it +3 Vote: I do not like it

Feels sad man , when first contest you give goes unrated :(

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

nowdays Long queue is common problem in cf. please do something MikeMirzayanov :(

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    5 weeks ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Custom invocation is also unusable during contest.

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5 weeks ago, # |
  Vote: I like it +27 Vote: I do not like it

Imagine waiting for a Codeforces round for 1 week... and this.

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5 weeks ago, # |
  Vote: I like it +38 Vote: I do not like it

Unlucky comeback after ~2 years of inactive. F.

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5 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

Please fix this!

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5 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

unraited...

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5 weeks ago, # |
  Vote: I like it +90 Vote: I do not like it

How to get rid of queues on CF?

The answer is...
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5 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

Unrated?

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5 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

comeback failed, folks. :(

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5 weeks ago, # |
Rev. 2   Vote: I like it 0 Vote: I do not like it

Could have been one of my best, really sad. But a lovely problem set. Cheers to the setters!

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5 weeks ago, # |
  Vote: I like it +24 Vote: I do not like it

It was the first contest I liked in probably the last 12 months and seeing it getting unrated this way, I feel sad for the authors very much. Markadiusz and Asymmetry I liked the problems a lot and hope to see you guys with another contest soon!

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5 weeks ago, # |
  Vote: I like it +52 Vote: I do not like it

Problem 1C seems to be the same with Problem C in 2020-2021 ACM-ICPC, Asia Kunming Regional Contest.

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5 weeks ago, # |
  Vote: I like it +3 Vote: I do not like it

cucked my comeback

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5 weeks ago, # |
Rev. 2   Vote: I like it 0 Vote: I do not like it

Problems was really nice. i think i will become specialist again after this contest :) but Unfortunately unrated :(

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5 weeks ago, # |
  Vote: I like it +41 Vote: I do not like it

The problems are nice! Because it is unrated now, I can sleep early today LOL :)

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

This is still a good problem collection and worth trying out. I will still try to do my best even if it's unrated.

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5 weeks ago, # |
  Vote: I like it -8 Vote: I do not like it

Can there be a announcement that they will rate the round? If not please tell me so that I can stop worrying. :(

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    5 weeks ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Don't Worry Codeforces always keep their words... They said its unrated means its unrated...

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5 weeks ago, # |
Rev. 2   Vote: I like it +52 Vote: I do not like it

1-gon's first round got unrated due to the long queue. Now Asymmetry and Markadiusz set their first round coordinated by 1-gon, and the round got unrated due to the long queue. RIP

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5 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

logic behind b?

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    5 weeks ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Since a[0]!=b[0] we need to make those two elements so that a[0]<b[0]

    That is, we iterate all a[i], and foreach one we find the smallest possible j so that a[i]<b[j] That pair of i,j need i+j operations, we search for the min such pair.

    We can find the smallest j for a given i by binary search on the prefix arrays of a[] and b[]. make preA[i] the smallest value in a[0..i], and preB[j] the biggest value in b[0..j]. Binary search the smallest j where preA[i]<preB[j].

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      5 weeks ago, # ^ |
      Rev. 2   Vote: I like it +3 Vote: I do not like it

      We can avoid binary search since we know the numbers in a[] are 1, 3, ..., 2n-1. Create an array c[] such that c[k] gives the index j for number k i.e. if a[i]=k, we get j by c[k]. To fill c[], we can iterate over array b and store index j for all numbers smaller than the current no in b.

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        5 weeks ago, # ^ |
          Vote: I like it 0 Vote: I do not like it

        Doesn't this require sorting, which yields linearithmic time complexity either way?

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          5 weeks ago, # ^ |
            Vote: I like it 0 Vote: I do not like it

          No, the numbers are relatively small compared to the length of the input (<=n), so a simple loop, described above, is enough.

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            5 weeks ago, # ^ |
              Vote: I like it +8 Vote: I do not like it

            Oh I completely misunderstood that comment. I got it now, thanks!

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5 weeks ago, # |
  Vote: I like it -49 Vote: I do not like it

give editorial ASAP.
I have to do a lot of homework.

Spoiler
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    5 weeks ago, # ^ |
      Vote: I like it +11 Vote: I do not like it

    well I think: 2020 -> 0022 -> 0020 -> 0002 -> 0000

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      5 weeks ago, # ^ |
      Rev. 3   Vote: I like it 0 Vote: I do not like it

      swap two digits (you can choose which digits to swap, and they don't have to be adjacent). how the following operation is feasible with one swap
      we must have to do two consecutive swap.
      0020 -> 2000 -> 0002

      please clarify

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        5 weeks ago, # ^ |
        Rev. 3   Vote: I like it 0 Vote: I do not like it

        swap 0 and 2?

        i don't understand what you wonder...

        p/s: they don't have to be adjacent, but adjacent is accepted

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          5 weeks ago, # ^ |
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          logic was sum up all non zero numbers and also increase by 1 if the position of nonzero element is not last...

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        5 weeks ago, # ^ |
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        i also have the same doubt becaz this---

        "you can choose which digits to swap, and they don't have to be adjacent" has been clearly mentioned in the problem

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    5 weeks ago, # ^ |
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    2020 -> 2002 (swap 3rd and 4th element) -> 2001 -> 2000 -> 0002 (swap 1st and 4th element) -> 0001 -> 0000

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The problems were good. (especially Problem B...able to do in O(N)) PS: All the best for Kickstart buddies. :)

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First time my code was accepted in contest but fine i enjoyed the problem....

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Any thoughts of problem E? It seems to be a dp problem, but I didn't figure out the recursion formula.

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    5 weeks ago, # ^ |
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    it looks more like a divide and conquer problem for me, i tried dp and it didn't work.

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    5 weeks ago, # ^ |
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    NVM. Thought it was D.

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I dont understand why people in comment section are filled with sarcasm and down votes

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For problem C, we first figure out if it is possible to finish the whole book by SCC (strongly connected component). If size of some (possibly 1) SCC are greater than 1, then it is impossible to complete the book. After that, we use 2 min heap to store pages with indegree 0. First heap stores current traverse. Second heap stores next traverse.

Link for my submission

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    5 weeks ago, # ^ |
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    Using SCC is a bit overkill and definitely a waste of time for this problem.

    The goal (when checking if understanding the book is possible) is to check if there is a cycle. We can do that with a DFS (or multiple, in case of a DAG, which we are dealing with here) which, instead of marking a vertex visited, can set two different kinds of flags.

    The DFS works as follows:

    1. Set flag "in progress" for current vertex
    2. Consider all neighbours, for each 2.1. If its flag is "in progress", then a cycle is detected 2.2. If its flag is "unvisited", then run the DFS on the neighbour.
    3. Set flag "visited" for current vertex

    We can count the result with a DFS too, using dynamic programming on a DAG. Every vertex with no outgoing edges receives value 1, because that chapter will be understood during the first reading of the book. Every other chapter will be understood after all its prerequisite chapters are understood, so the result for the chapter is the maximum of the results for the prerequisite chapters... but slightly modified to account for the order of the chapters in the book.

    I'm not the best at explaining so maybe my code will help.

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      5 weeks ago, # ^ |
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      topological sort :|

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      I actually used a different approach. I made two heaps : the first one is the "main" heap, the second one is the "pending" heap

      The "main" heap's purpose is to store the progress chapters you can learn right now given that you're already learn all required chapters. Meanwhile, the "pending" heap's purpose is to store the chapters which you have learned all the required chapters but the chapter number is less than the chapter we currently on

      So let's process the chapters one by one starting by those which have no requirements. (E.g. the ones which have 0 in-degree)

      Each time we're processing of the current chapter (let's say we name it U) we do this :

      • check all other chapters V which has U as requirements

      • subtract V's degree by 1

      • if V's degree become 0 then : A. If V > U, then this chapter comes after U, then put it on the "main" heap OR B. If V < U, then this chapter lies before U, since we can't read it backwards, put V to "pending" heap

      on the end of each processing, if our "main" heap becomes empty, put all chapters in "pending" heap to "main" heap and increase the number of read by 1 (because this means we will start re-reading the book)

      To check if it's impossible, just check if after we're done with all the processing, there exists at least a chapter we haven't processed before

      My implementation

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        5 weeks ago, # ^ |
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        This work is O(nlog(n)) though due to the heap! But it's still a fine approach.

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How to solve div2 D?

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    5 weeks ago, # ^ |
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    First of all, xor of all the elements of the array should be $$$0$$$ (xor of all the elements of the array isn't changed after any operation).

    From here, I am assuming that xor of all the elements of the array is $$$0$$$.

    If n is odd, it's always possible to change all the elements to $$$0$$$.

    Note that if $$$a_{i}=0$$$ and $$$a_{i+1}=a_{i+2}$$$, $$$a_{i}\oplus a_{i+1}\oplus a_{i+2}=0$$$. This is the main idea of my approach.

    Now select indices $$$n-2,n-4,...,1$$$. It's easy to observe that now the first element of the array will be $$$0$$$ and $$$a_{i}=a_{i+1}$$$ for all even $$$i$$$ less than $$$n$$$. First element will always be $$$0$$$ because it is the xor of all the elements of the array Now select $$$1,3,...n-2$$$. After these operations, all the elements of the array will be $$$0$$$ and total number of operations performed is $$$n-1$$$.

    If $$$n$$$ is even, you can split it into two subarrays of odd length. So if you can find an $$$i$$$ such that $$$i$$$ is odd and xor of first $$$i$$$ elements is $$$0$$$, you can change all the elements to $$$0$$$, otherwise you cannot.

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      5 weeks ago, # ^ |
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      Oh holy crap this is brilliant. And here I got stuck with some greedy ifological strategy which... I'm reasonably confident would work but I didn't manage to implement it.

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        5 weeks ago, # ^ |
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        Yeah, such approach can work too but I had to consider many different cases.

        Basically: we iterate from left to right, either A keeping all elements behind 0 or B leaving all elements behind 1 (for simplicity ensuring that the current element is 1 too after previous operations).

        A: if applying the operation to the current $$$i$$$ will result in zero xor, we do it. Else if the $$$i$$$th element is 1, we ensure that $$$i+1$$$ is too; we know that $$$i-1$$$, if it exists, is zero, and we can zero $$$i-1..i+1$$$, and if it doesn't, we switch to B.

        B: if applying the operation to the current $$$i$$$ will result in zero xor AND there's an even number of 1s behind us, we zero $$$i..i+2$$$ and 1s behind us two at a time. Otherwise we ensure that the next element is 1 (working through a few cases it can be shown that if it's 0, the current xor must be 1).

        code
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    5 weeks ago, # ^ |
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    First you must have an even number of 1s in the array, otherwise it's impossible to change all elements to 0s.

    Then group all the 1s in pairs: (1st, 2nd), (3rd, 4th), ... we will solve the problem by changing each pair of 1s to 0s repeatedly:

    1. if there are an even number of 0s between two 1s, we firstly change those 0s to 1s by performing operations on i such that $$$a_i=1, a_{i+1}=0, a_{i+2}=0$$$ repeatedly. Then perform operations on i such that $$$a_i=0, a_{i+1}=1, a_{i+2}=1$$$. Note that assume the indices of the two 1s are $$$l$$$ and $$$r$$$, we must have either $$$a_{l-1}=0$$$ or $$$a_{r+1}=0$$$ in order to solve this case.
    2. if there are an odd number of 0s between two 1s, similar to case 1, we first change all zeros to ones except for the rightmost zero. Now we have $$$1\dots 101$$$, then we perform an operation on 101, then do the similar thing on case 1 but from right to left. Note that there are no extra requirement to this case.

    So the strategy is firstly find one pair that can be solved then solve the remaining pairs from that pair in both direction. If none of the pairs can be solved, it's impossible to change all 1s to 0s. Submission: 129221560

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https://codeforces.com/contest/1573/submission/129205633 Can someone please help me in figuring out why this submission gets TLE?

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How to solve div2 B ?

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    5 weeks ago, # ^ |
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    since array a is odd numbers and array b is even numbers.
    so lexicographically smaller/greater is defined at index 0.
    so only comparing values at index 0 is enough
    
    
    let cost[x] = # swaps to move a value v < x in array 'a' to index 0.
    
    ans = 2n
    and iterate over i = 0 to n - 1:
      ans = min(ans, i + cost[b[i]]); // i <- #swaps to move i_th number in array b to index 0
    
    if iterating is over.
    the ans will be the answer
    
    ----------------
    how to create cost array?
    intialize cost array with max value = n
    for (int i = 0; i < n; ++i) {
        cost[a[i]] = i;
    }
    for (int i = 1; i < 2 * n; ++i) {
       cost[i] = min(cost[i], cost[i - 1]);
    }
    
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    5 weeks ago, # ^ |
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    We need to find the minimum no of swaps so that no at the first index of odd array is less than the no at the first index of the even array . One way to do this is brute force. For each even no we try to get the nearest odd no which is less than that even no in the odd array. But this will be a O(n*n) algorithm and may give TLE for the tight constraints. So a better way is to store the indices in some ds like a map and for each no 2 4 6 8 ... We store the minimum of the indices of the current no — 1 and the index of previous no .And finally itterate through all the possibilities which will be only n checks and get the global minimal answer out of all the possible.

    My Accepted code :https://codeforces.com/contest/1573/submission/129208921

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Although round got unrated but I liked task B!

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My segment tree solution for Div1 A

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    5 weeks ago, # ^ |
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    can you just briefly explain what you are storing/updating in the nodes?

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Can anyone help me to find a counter case? 129205285

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    5 weeks ago, # ^ |
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    1
    4
    3 2 3 4
    0
    2 2 4
    0
    

    Correct answer is 3 yours gives 2.

    I have used a similar approach with the topological sort, except instead of storing the chapters to be read in the next iteration in vec, i just added n to the chapter and pushed it into pq. 129229753

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      5 weeks ago, # ^ |
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      Thanks a lot, but i was so dumb that i forget priority queue stores greater elemnet first. I thought it keeps smaller element first. You just saved a lot of time mine.

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I have tried a video editorial for problem C :book https://youtu.be/KWkkZEGffZw . Have a look at it..and if have any doubts let me know in comments

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Can I change the statement that a[i] is (0/1) to [0,100000] on Problem 'Xor of 3'.Is there a solution?

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    5 weeks ago, # ^ |
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    Maybe you can solve the task in each bit, but the operation times may exceed $$$n$$$.

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      I think We can do that as well in n-1 operations.

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    I think my solution doesn’t use the 0/1 condition so probably yes.

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editorial is also still ``in queue'' LOL

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I'm newbie :(((

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For Question B, Can Someone please tell me why my approach is wrong? https://codeforces.com/contest/1573/submission/129231370

My approach is that our task is to make a[0] < b[0], this can be done by two ways, either bring the closest number which is greater than a[0] in 'b' to the 0th position, let's say it is at j-index, then the answer will be 'j', or bring the closest element from 0 in a which is less than b[0], and the final answer is the minimum of both!

Any counter test-case which fails this approach would be highly appreciated!

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You want the clock to show 0 with as few operations as possible. In an operation, you can do one of the following:

1 decrease the number on the clock by 1, or

2 swap two digits (you can choose which digits to swap, and they don't have to be adjacent).

here in 2nd operation we cannot swap adjecent digits but no one is talking about it ! plzz reply am not getting the exact thing or it is something that was some mistake from creators and now no one is talking about it since they got ac?

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Is div1 B supposed to make us feel pain?

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Easy solution for B [Sparse Table]: 129189027

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waiting for editorial ...

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orz Molewus for the round being unrated.

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