maksyuki 发表于 oj 分类,标签: ,

Going from u to v or from v to u?

In order to make their sons brave, Jiajia and Wind take them to a big cave. The cave has n rooms, and one-way corridors connecting some rooms. Each time, Wind choose two rooms x and y, and ask one of their little sons go from one to the other. The son can either go from x to y, or from y to x. Wind promised that her tasks are all possible, but she actually doesn't know how to decide if a task is possible. To make her life easier, Jiajia decided to choose a cave in which every pair of rooms is a possible task. Given a cave, can you tell Jiajia whether Wind can randomly choose two rooms without worrying about anything?


The first line contains a single integer T, the number of test cases. And followed T cases.

The first line for each case contains two integers n, m(0 < n < 1001,m < 6000), the number of rooms and corridors in the cave. The next m lines each contains two integers u and v, indicating that there is a corridor connecting room u and room v directly.


The output should contain T lines. Write 'Yes' if the cave has the property stated above, or 'No' otherwise.

Sample Input


3 3

1 2

2 3

3 1

Sample Output



POJ Monthly--2006.02.26,zgl & twb


题目类型:有向图强连通分量(Tarjan) +缩点+拓扑排序判定