Best problems of 2017

I have reviewed all problems I've highlighted this year in my blog, and also the problems sent by the readers in response to my question — thanks a lot for your input! I've distilled everything to a shortlist of five problems (in chronological order):

• The Open Cup problem about interactively proving the pigeonhole principle, by tunyash, with solution in this post.
• The AtCoder problem about moving two chips on a subset of a DAG, by sugim48, with solution in this post.
• The IPSC problem about picking optimal sequences of red and black cards to get the first match, by misof, with solution in this post.
• The AtCoder problem about assigning numbers to shared vertices of two trees so that all subtrees sum to +-1, by maroonrk, with solution in this post.
• The Open Cup problem about interactively exploring a maze on Mars with a communication delay, by Gassa, discussed in this post. 

What is the best problem of 2017?

As usual around New Year, snarknews is hosting the Prime New Year contest, which consists of problems given at top-level team competitions in 2017 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 :)

Guten Rutsch!