题目:hdu 1796, hdu 1685,hdu 2204,hdu 4407 ,hdu 2841
http://acm.swust.edu.cn/oj/problem/821/
http://zuojie.3322.org:88/soj/problem.action?id=3137
http://acm.hdu.edu.cn/showproblem.php?pid=1796
http://acm.hdu.edu.cn/showproblem.php?pid=1695
http://poj.org/problem?id=3904
hdu 2841
http://yzmduncan.iteye.com/blog/1315175
http://www.cnblogs.com/scau20110726/archive/2013/03/18/2966089.html