1209  Playing games
Time Limit : 1 Second
Memory Limit : 128 MB
Submission: 133
Solved: 63
 Description
 In our childhood, we all like playing games with our friends. Now you are a teacher in a kindergarten and there are n children in your class. Before playing games, you must divide the children into some small groups and each group must contains more than one child, but a child only wants to stand besides his or her friends. k (k > 1) children can form a group only if there exists a circular permutation of k children in which each child only stand besides his or her friends.
To make the problem simple, given you n children and the relationship between them, you must judge if every child can be divided into some group and no one will be left alone.
 Input
 In the first line of the input, there is an integer T (0 < T <= 20), which means T test cases.
For each test case, the first line contains two integers n (0 < n <= 400) and m (0 < m < n*n/2), which are the number of the children and the number of the relationship. There are m lines followed, every line has two integers a and b (1 <= a, b <= n ), means child a and child b are friends..
 Output
 For each test case, if every child can be divided into some group and no child will be left alone, output "YES", or else output "NO".
 sample input

2 4 2 1 2 3 4 3 2 1 2 1 3
 sample output

YES NO
 hint
 source