HDU1272 小希的迷宫

用并查集判断图是否连通，以及是否存在环~

#include<cstdio>
#include<algorithm>
#include<cstring>
#include<vector>
#include<unordered_map>
using namespace std;
const int maxn=1e6+14;
const int inf=1e9;
int father[maxn],isRoot[maxn],flag,visit[maxn];
void init () {
    for (int i=0;i<maxn;i++) father[i]=i;
    flag=0;
    fill (isRoot,isRoot+maxn,0);
    fill (visit,visit+maxn,0);
}
int findfather (int x) {
    int a=x;
    while (x!=father[x]) x=father[x];
    while (a!=father[a]) {
        int z=a;
        a=father[a];
        father[z]=x;
    }
    return x;
}
void Union (int a,int b) {
    int faA=findfather(a);
    int faB=findfather(b);
    if (faA!=faB) father[faA]=faB;
}
int main () {
    while (1) {
        int flag=0,x,y;
        init ();
        while (1) {
            scanf ("%d %d",&x,&y);
            if (x==0&&y==0) break; 
            if (x==-1&&y==-1) return 0;
            if (findfather(x)==findfather(y)) flag++;
            visit[x]=1;visit[y]=1;
            Union (x,y);
        }
        for (int i=0;i<maxn;i++) if (visit[i]) isRoot[findfather(i)]++;
        int ans=0;
        for (int i=0;i<maxn;i++) if (isRoot[i]&&visit[i]) ans++;
        if (ans>1) flag++; 
        //printf ("%d\n",ans);
        if (flag==0) printf ("Yes\n");
        else printf ("No\n");
    }
    return 0;
}