Saturday, April 11, 2020

A topology week

The Mar 16 - Mar 22 week had competitions from the three main platforms. Codeforces was the first to go, with its Global Round 7 on Thursday (problems, results, top 5 on the left, analysis). Um_nik was the first to finish solving the problems, but tourist had beaten him to the first place because he solved the problem F1, which is a reduced-constraint version of problem F2, separately and much faster, while Um_nik just solved F2 directly and got F1 accepted at the same time with a lower score. Congratulations to both!

TopCoder SRM 781 followed in the middle of the night on Friday (problems, results, top 5 on the left, analysis). My lead in the TCO qualification before this round was considerable at 31 points to 24, so I've decided to not wake up at 2 in the morning, even though mathematically I could still lose my spot if I do really badly in SRM 782. Congratulations and big thanks to Egor who made sure the five points did not go to one of the 24-point pursuers :)

Finally, AtCoder Grand Contest 043 marked the start of a new season there (problems, results, top 5 on the left, analysis). apiad started with the hardest problem F and got it accepted with 47 minutes remaining in the contest, probably realizing that others are not likely to get it. There were therefore a few paths to the first place open for him. Solving B+C+D in 47 minutes was probably the most realistic by looking at the scoreboard, but he went for the very interesting and unusual problem E (D+E would be enough for the first place) and didn't manage to get it in time. tourist was the fastest of those on a more traditional problem solving order, and won the round with a 11-minute gap. Well done!

There were several nice problems in this round, so let me highlight an easier problem for a change, problem B: you are given a sequence of n<=106 numbers, each number is 1, 2 or 3. Then, you create a new sequence of length n-1 by taking the absolute values of the differences between adjacent numbers. Then you create a new sequence of length n-2 from the sequence of length n-1 in the same way, and so on until there's just one number in the sequence. What will this number be?

Thanks for reading, and check back for more!