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?

输入格式

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.

输出格式

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

题目大意

从u到v还是从v到u？

题目描述

为了让儿子们变得勇敢，Jiajia 和 Wind 带他们去了一个大山洞。这个洞穴有 $n$ 个房间，还有连接一些房间的单向走廊。每次，Wind 都会选择两个房间 $x$ 和 $y$，并让他们的一个小儿子从一个房间走到另一个房间。儿子可以从 $x$ 到 $y$，也可以从 $y$ 到 $x$。Wind 承诺她的任务都是可能的，但她实际上不知道如何决定一个任务是否可能。为了让她的生活更轻松，Jiajia 决定选择一个洞穴，每一对房间都是一个可能的任务。给定一个洞穴，你能告诉 Jiajia 和 Wind 是否可以随意选择两个房间而不必担心任何事情吗？

输入格式

第一行包含一个整数 $T$ ，即测试用例的数量。 每种情况的第一行包含两个整数 $n，m$$（0<n<1001，m<6000）$，洞穴中的房间和走廊数量。接下来的$m$行各包含两个整数 $u$ 和 $v$，表示有一条走廊直接连接房间 $u$ 和房间 $v$。

输出格式

输出应包含 $T$ 行。如果洞穴具有上述属性，请写“Yes”，否则写“No”。

1 
3 3 
1 2 
2 3 
3 1

Yes