題目描述：
ACM-DIY is a large QQ group where many excellent acmers get together. It is so harmonious that just like a big family. Every day,many "holy cows" like HH, hh, AC, ZT, lcc, BF, Qinz and so on chat on-line to exchange their ideas. When someone has questions, many warm-hearted cows like Lost will come to help. Then the one being helped will call Lost "master", and Lost will have a nice "prentice". By and by, there are many pairs of "master and prentice". But then problem occurs: there are too many masters and too many prentices, how can we know whether it is legal or not?We all know a master can have many prentices and a prentice may have a lot of masters too, it's legal. Nevertheless，some cows are not so honest, they hold illegal relationship. Take HH and 3xian for instant, HH is 3xian's master and, at the same time, 3xian is HH's master,which is quite illegal! To avoid this,please help us to judge whether their relationship is legal or not. Please note that the "master and prentice" relation is transitive. It means that if A is B's master ans B is C's master, then A is C's master.
輸入：
The input consists of several test cases. For each case, the first line contains two integers, N (members to be tested) and M (relationships to be tested)(2 <= N, M <= 100). Then M lines follow, each contains a pair of (x, y) which means x is y's master and y is x's prentice. The input is terminated by N = 0.TO MAKE IT SIMPLE, we give every one a number (0, 1, 2,..., N-1). We use their numbers instead of their names.
輸出：
For each test case, print in one line the judgement of the messy relationship.If it is legal, output "YES", otherwise "NO".
樣例輸入：
3 2
0 1
1 2
2 2
0 1
1 0
0 0
樣例輸出：
YES
NO

 大意：master 師傅、prentice 徒弟
一個qq群，一些人之間存在師徒關係，該關係是傳遞的。讓判斷是否有兩人，存在既是師徒，又是徒弟跟師傅這樣的非法關係。

分析：用拓撲排序來解決。每人看做一個點，a是b的師傅，則用相應有向弧存儲。