HDU1272-小希的迷宫

http://acm.hdu.edu.cn/showproblem.php?pid=1272

#include<iostream>
#define MAX 100005
using namespace std;
int fa[MAX],flag,sign[MAX];
int find(int x)
{
    return(fa[x]==x?x:find(fa[x]));
}
void merge(int x,int y)
{
     x=find(x);
     y=find(y);
     if(x!=y)
        fa[x]=y;
     else 
        flag=0;
}
int main(void)
{
    int i,a,b,k;
    while(cin>>a>>b)
    {
        if(a==-1&&b==-1) 
           break;
        if(a==0&&b==0)
        {
            cout<<"Yes"<<endl; 
            continue; 
        }
        for(i=1;i<MAX;i++) 
        {
             fa[i]=i;
             sign[i]=0;
        }
        sign[a]=sign[b]=1;
        flag=1;
        merge(a,b);
        while(cin>>a>>b)
        {
            if(a==0&&b==0) 
               break;
            merge(a,b);
            sign[a]=sign[b]=1;
        }
        k=0;
        for(i=1;i<MAX;i++)
        {
            if(sign[i]&&fa[i]==i) 
               k++; 
            if(k>1) 
               flag=0;
        }
        if(flag) 
           cout<<"Yes"<<endl;
        else 
           cout<<"No"<<endl;
     }
     return 0;
}