2014.02.23
http://acm.timus.ru/problem.aspx?space=1&num=1212
http://acm.fzu.edu.cn/problem.php?pid=1608
2014.02.26
http://codeforces.com/problemset/problem/6/D
http://codeforces.com/problemset/problem/6/E
原文：http://www.cnblogs.com/sineatos/p/3570502.html