Best problems of 2018

Just like last year, I have reviewed the problems I've highlighted in this blog, and have picked the shortlist of five problems that I think were the best in 2018 (in chronological order):

• The AtCoder problem about traversing the vertices of a 0/1-labeled tree minimizing the number of inversions, by maroonrk, with solution in this post.
• The Codeforces problem about rearranging an array in such a way that prefix xors are increasing, by Endagorion, with solution in the same post. 
• The AtCoder problem about two players taking numbers from an array, where one must take adjacent number when available, by sugim48, with solution in this post.
• The Open Cup problem about rolling a ball in a maze while collecting all targets in some order, by jh05013, with solution in this post.
• The TopCoder problem about creating a figure with exactly n domino tilings, by Alex_2oo8, with solution in this post.

Which one do you think is the very best?

There are other discussions and votes about the best problem of 2018, check those out :) In that second post, snarknews is also announcing the Prime New Year contest, which consists of problems given at top-level team competitions in 2018 but not solved by anybody there. If you like real challenge and don't have much to do during the holidays, here's your link.

Happy New Year!