qoo2p5's blog

By qoo2p5, history, 3 months ago, translation, In English,
Problem A. Key races
Problem B. The number on the board
Problem C. Star sky
Problem D. Palindromic characteristics
Problem E. The penguin's game
Problem F. Roads in the Kingdom

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By qoo2p5, 3 months ago, translation, In English,


On Monday, July 31, 2017, at 14:35 UTC rated Codeforces Round #427 for participants from the second division will take place. As always, participants from the first division can take part out of competition.

The problems for this round were prepared by me. Many thanks to Alexey Ilyukhov (Livace) for help in preparations of the round and testing the problems, AmirReza PoorAkhavan (Arpa) for proofreading the statements and testing the problems, Gaev Alexandr (krock21) for testing the problems, Nikolay Kalinin (KAN) for the round coordination and, of course, Mike Mirzayanov (MikeMirzayanov) for great Codeforces and Polygon platforms.

The round will last for 2 hours, and you will be given 6 problems. I recommend you to read the statements of all problems. I hope everyone will find an interesting problem!

Scoring will be announced before the round.

Scoring: 500 — 750 — 1250 — 1500 — 2250 — 2250.


Thanks for participating!

The editorial is here.

Congratulations to the winners!


  1. ywwyww

  2. nick452

  3. JustAnAverageCoder123

  4. cxt

  5. wa1tz7I9


  1. dotorya

  2. rajat1603

  3. anta

  4. Kaban-5

  5. HellKitsune

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By qoo2p5, 4 months ago, In Russian,


Допустим, есть множество из n элементов a1, ..., an, на котором введено отношение частичного порядка  < , притом у нас есть оракул, который позволяет узнать: верно ли, что ai < aj. Необходимо отсортировать данные элементы, то есть найти такую перестановку b1, ..., bn, что не найдётся i < j таких, что bj < bi.

Хочется минимизировать количество обращений к оракулу. Понятно, что можно сделать O(n2) обращений, используя топологическую сортировку. Но можно ли делать это быстрее? Если нет, то интересно узнать доказательство этого.

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By qoo2p5, 5 months ago, In English,

I want to ask people who has already prepared Codeforces contests: how long had you been waiting for a response?

I have no response for more than 6 weeks... It's really disappointing.

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