LA 3644并查集，图的联通分量

元素看成顶点，化合物看成一条边。用并查集维护图的联通分量。对于每个新增的化合物（x, y）,如果x和y在同一个联通分量里，则加入边（ｘ，ｙ）使得联通分量形成环，从而危险。

代码如下：

#include <cstdio>
const int maxn=1e6;
int pa[maxn];
int find(int x){return pa[x] != x ? pa[x]=find(pa[x]) : pa[x];}
int main(){
    int x, y;
    while(scanf("%d",&x)==1){
        for(int i=0;i<maxn;i++){pa[i] = i;}
        int refusals = 0;
        while (x != -1){
            scanf("%d",&y);
            x = find(x);y = find(y);
            if (x==y) refusals++;//x and y have been in the same compent, then adding(x,y) will cause a cycle
            else pa[x] = y;
            scanf("%d",&x);
        }
    printf("%d\n",refusals);
    }
    return 0;
}