https://www.lydsy.com/JudgeOnline/problem.php?id=3498
https://cn.vjudge.net/problem/HDU-5977
https://www.lydsy.com/JudgeOnline/problem.php?id=1125
https://www.lydsy.com/JudgeOnline/problem.php?id=3626
https://www.lydsy.com/JudgeOnline/problem.php?id=4108
https://www.lydsy.com/JudgeOnline/problem.php?id=4078
https://blog.csdn.net/zmoiynlp/article/details/45175049
http://poj.org/problem?id=3764