Ice_cream's world I HDU - 2120 

              ice_cream's world is a rich country, it has many fertile lands. Today, the queen of ice_cream wants award land to diligent ACMers. So there are some watchtowers are set up, and wall between watchtowers be build, in order to partition the ice_cream’s world. But how many ACMers at most can be awarded by the queen is a big problem. One wall-surrounded land must be given to only one ACMer and no walls are crossed, if you can help the queen solve this problem, you will be get a land.InputIn the case, first two integers N, M (N<=1000, M<=10000) is represent the number of watchtower and the number of wall. The watchtower numbered from 0 to N-1. Next following M lines, every line contain two integers A, B mean between A and B has a wall(A and B are distinct). Terminate by end of file.OutputOutput the maximum number of ACMers who will be awarded. 
One answer one line.Sample Input：

8 10
0 1
1 2
1 3
2 4
3 4
0 5
5 6
6 7
3 6
4 7

Sample Output：

3
寻找集合中环的个数！

#include<cstdio>
#include<string>
#include<iostream>
#include<algorithm> 
#define MAX 1005
using namespace std;
 
int fa[MAX];
int N,M,cnt;

int find(int u)
{   if(u!=fa[u]) fa[u]=find(fa[u]);
    return fa[u];    
}

void Union(int a,int b)
{   if(find(a)==find(b)) cnt++;
    else fa[find(b)]=find(a);
}

int main()
{   int k,n,m;
    while(cin>>N>>M)
    {   for(int i=0;i<=MAX;i++) fa[i]=i;  
        for(k=0;k<M;k++)
        {   scanf("%d%d",&n,&m);
            Union(n,m);
        }
        cout<<cnt<<endl;
        cnt=0;
    }
}