Tree or Not

Difficulty: 1300 4000 ms 128 MB

graphs trees

Is the given undirected graph a tree (connected and cycle-free)?

Input: the first line has N and M; each of the next M lines has an edge u v (1-based).

Output: YES or NO.

Constraints:

• 1 ≤ N ≤ 10⁴
• 0 ≤ M ≤ 2×10⁴
• multiple edges and self-loops may appear

Sample Tests

4 3
1 2
2 3
2 4

YES

4 3
1 2
2 3
1 3

NO

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