http://acm.nudt.edu.cn/~twcourse/TransitiveClosure.html
https://en.wikipedia.org/wiki/Closure_(mathematics)#P_closures_of_binary_relations
http://cs.winona.edu/lin/cs440/ch08-2.pdf
https://en.wikipedia.org/wiki/Closure_(mathematics)#P_closures_of_binary_relations
原文：http://www.cnblogs.com/WCFGROUP/p/6284372.html