PT07Y - Is it a tree
You are given an unweighted, undirected graph. Write a program to check if it's a tree topology.
The first line of the input file contains two integers N and M --- number of nodes and number of edges in the graph (0 < N <= 10000, 0 <= M <= 20000). Next M lines contain M edges of that graph --- Each line contains a pair (u, v) means there is an edge between node u and node v (1 <= u,v <= N).
Print YES if the given graph is a tree, otherwise print NO.
Input: 3 2 1 2 2 3 Output: YES