Submit ID is 1027752.

For test
5 4
1 2
2 3
1 3
4 5

My answer is NO.

But you can number flights: 1 2 3 4  :)

My solution should be OK for connected graphs. But the test set lacks for disconnected cases.

read the task carefully

I have re-read and could't find my mistake. Can you help me?

And I've found the case of connected graph with WA.

5 6
1 2
1 4
2 4
3 4
4 5
3 5

My solution's answer is NO, but 2 1 3 4 6 5 seems to be OK.

So the test set of Timus is really incomplete.