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并查集判环:

#include<stdio.h>
#include<iostream>
#include<math.h>
#include<stdlib.h>
#include<ctype.h>
#include<algorithm>
#include<vector>
#include<string.h>
#include<queue>
#include<stack>
#include<set>
#include<map>
#include<sstream>
#include<time.h>
#include<utility> 
#include<malloc.h> 
#include<stdexcept>

using namespace std;

int n,m;
int p[1000000];
int num;

int find (int x)
{
    if (x == p[x])
        return x ;
    else return find (p[x]); 

}

void un(int a ,int b)
{
    int fa = find (a);
    int fb = find (b);
    if (fa==fb)
    {
        num++;
        return ;

    }
    else
    {
        if (fa>fb)
            p[fa]=fb;
        else 
            p[fb]=fa;
    }
}

int main ()
{
    int n,m;

    while (scanf ("%d%d",&n,&m)!=EOF )
    {
        num=0;
        for (int i=0;i<n;i++)
            p[i]=i;
        int a,b;
        for (int i=0;i<m;i++)
        {
            scanf ("%d%d",&a,&b);
            un(a,b);
        }

        printf("%d\n",num);
    }
    return 0;
}