Going from u to v or from v to u?
Time Limit:2000MS Memory Limit:65536K
Total Submit:2209 Accepted:290
Description 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?
Input 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.
Output The output should contain T lines. Write 'Yes' if the cave has the property stated above, or 'No' otherwise.
Sample Input
1
3 3
1 2
2 3
3 1
Sample Output
Yes
Source POJ Monthly--2006.02.26,zgl & twb
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