戻すDP
以下、未整理
ARC 070 D. No Need
http://arc070.contest.atcoder.jp/tasks/arc070_b
https://codeforces.com/contest/1111/problem/D
https://community.topcoder.com/stat?c=problem_statement&pm=11026
http://mayokoex.hatenablog.com/entry/2016/01/03/165804
http://kmjp.hatenablog.jp/entry/2014/12/19/1030
http://kmjp.hatenablog.jp/entry/2014/12/23/0930
http://kmjp.hatenablog.jp/entry/2015/08/01/1030