PT07Y  Is it a tree
You are given an unweighted, undirected graph. Write a program to check if it's a tree topology.
Input
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).
Output
Print YES if the given graph is a tree, otherwise print NO.
Example
Input: 3 2 1 2 2 3 Output: YES
hide comments
kiransk:
20110422 04:44:35
is the input graph connected? 

albertg:
20110419 07:03:38
YES. THAT'S RIGHT. 

:D:
20110117 21:45:39
One test case per file. 

Marko Ivankovic:
20090821 08:15:54
Read about it here: http://en.wikipedia.org/wiki/Tree_(graph_theory) Last edit: 20090821 08:16:22 
Added by:  ThanhVy Hua 
Date:  20070328 
Time limit:  0.5s 
Source limit:  50000B 
Memory limit:  1536MB 
Cluster:  Cube (Intel G860) 
Languages:  All except: ERL JSRHINO 
Resource:  Coauthor Amber 