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Legal or Not
Limit: 2000/1000 MS
Limit: 32768/32768 K (Java/Others)
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
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
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
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.
For each test case, print in one line the judgement of the messy
If it is legal, output "YES", otherwise "NO".