6 8  5 3  5 2  6 4
5 6  0 0

8 1  7 3  6 2  8 9  7 5
7 4  7 8  7 6  0 0

3 8  6 8  6 4
5 3  5 6  5 2  0 0

-1 -1

Yes
Yes
No


The code；
#include<stdio.h>  
const int N = 100005;  
int flag[N], father[N];   
void Init()  
{  
    for(int i = 0; i <= 100000; i++)  
        flag[i] = 0, father[i] = i;  
}  
int Find(int x)  
{  
    if(x != father[x])  
        father[x] = Find(father[x]);  
    return father[x];  
}  
void Merge(int a, int b)  
{  
    int p = Find(a);  
    int q = Find(b);  
    if(p > q)  
        father[p] = q;  
    else  
        father[q] = p;  
} 
int main()  
{  
    int a, b;  
    while(~scanf("%d%d",&a,&b))  
    {  
  
        if(a == -1 && b == -1)  
            break;  
        Init();  
        int FLAG = 0;
        while(1)
        {  
            if(a == 0 && b == 0)  
                break;  
            if(Find(a)  ==  Find(b))  
                FLAG = 1;  
            Merge(a,b);  
            flag[a] = 1, flag[b] = 1;  
            scanf("%d%d",&a,&b);  
        }  
        if(FLAG)  
            printf("No\n");  
        else  
        {
            int sum = 0;  
            for(int i = 0; i <= 100000; i++)  //记录下有几棵树； 
                if(flag[i] && father[i] == i)  
                    sum++;  
            if(sum  >  1)  //如果不止一棵树, 则输出No； 
                printf("No\n");  
            else  
                printf("Yes\n");
        }  
    }  
    return 0;  
}  
//通过flag数组与father数组，比较方便地记录下了树的棵数！