http://acm.hdu.edu.cn/showproblem.php?pid=6706
http://acm.hdu.edu.cn/showproblem.php?pid=6607
https://nanti.jisuanke.com/t/40522
https://nanti.jisuanke.com/t/A2003
https://nanti.jisuanke.com/t/39269
https://nanti.jisuanke.com/t/41390
https://blog.csdn.net/y_marcel_h/article/details/90225382
https://www.cnblogs.com/cjyyb/category/1124706.html