和上一道小希的迷宫差不多，但是在HDU上提交一直WA，POJ过了

HDU的数据太强了吧,强的我感觉数据有问题

题意：输入若干对点，判断是否是一颗树，转化过来也就是是否存在环

点数-边数=1，还要判断每个点的入度都<=1

POJ AC代码

#include <iostream>
#include <cstdlib>
#include <cstdio>
#include <cstring>
#include <algorithm>
const int INF = 1e8;
using namespace std;
int father[1010];
bool vis[1010];
int rudu[1010];
int findx(int r)
{
    int i = r,j;
    while(father[r]!=r)
    {
         r=father[r];
    }

    while(father[i]!=r)
    {
        j = father[i];
        father[i] = r;
        i = j;
    }
    return r;
}

bool Merge(int x,int y)
{
    int fx,fy;
    fx=findx(x);
    fy=findx(y);
    if(fx!=fy)
	{
	 father[fx]=fy;
	 return 1;
	}
	else
	return 0;
}
void init()
{
    for(int i=0;i<1010;i++)
		{
		father[i]=i;
		vis[i] = 0 ;
		rudu[i] = 0;
		}
}
int main()
{
	int a,b,c = 0;
	while(scanf("%d%d",&a,&b)!=EOF)
	{
	    c++;

	    if(a<0&&b<0)
            break;
	    int	flag=1,t=0;
         if(a==0 && b==0)
        {
            printf("Case %d ",c);
            puts("is a tree.");
            continue;
        }
		init();
	    int	num = 0;
		while(1)
		{
		    if(a==0&&b==0) break;
		    if(flag==1)
            {
			if(!vis[a]) {num++; } 
			if(!vis[b]) {num++; rudu[b]++;}
			if(rudu[b]>1)
                flag = 0;
			vis[a]=1; vis[b]=1;
           if(Merge(a,b)==1)
           {
                t++;    
           }
            else
                flag = 0;
            }
		   scanf("%d%d",&a,&b);
		}
		printf("Case %d ",c);
			if(num-t==1 &&flag == 1)
				puts("is a tree.");
			else
				puts("is not a tree.");
	}
	return 0;
}