问题 Z: Going from u to v or from v to u
内存限制:128 MB
时间限制:2.000 S
评测方式:文本比较
命题人:
提交:2
解决:2
题目描述
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?
输入
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 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.
输出
The output should contain T lines. Write 'Yes' if the cave has the property stated above, or 'No' otherwise.
样例输入 复制
1
3 3
1 2
2 3
3 1
样例输出 复制
Yes