https://cn.vjudge.net/problem/UVA-1160

只要該化合物的兩個分量已經屬於屬於同一強連通分量，化合物就不穩定，直接拒絕

#include<iostream>
#include<cstdio>
#include<algorithm>
using namespace std;
#define maxn 500005
int father[maxn];
int find(int x)
{
    if(father[x]!=x)
    father[x]=find(father[x]);
    return father[x];
}
void unionn(int x,int y)
{
    int r1=find(x);
    int r2=find(y);
    if(r1!=r2)
    father[r2]=r1;
}
int main()
{
    int x,y;
    while(~scanf("%d",&x))
    {
        if(x==-1)
        {
            printf("0\n");
            continue;

        }
        int cnt=0;
        for(int i=0;i<maxn;i++)
        father[i]=i;
        while(x!=-1)
        {
            scanf("%d",&y);
            if(find(x)==find(y))
            cnt++;
            else
            unionn(x,y);
            scanf("%d",&x);
        }
        printf("%d\n",cnt);
    }
    return 0;
}