代码如下  杭电的OJ能过. 这里却CE CE解释还看不懂..

#include<stdio.h>
#define maxn 1001
int i,j,father[maxn];
bool flag[maxn];
void makeset()
{
	for(i=0;i<1001;i++)
	{
		father[i]=i;
		flag[i]=0;
	}
}
int findfather(int x)
{
    if(x!=father[x])
        father[x]=findfather(father[x]);
    return father[x];
}
void Union(int a,int b)
{
    int x,y;
    x=findfather(a);
    y=findfather(b);
    if(x!=y)  father[x]=y;
}
int main()
{
    int a,b,max,min,cnt,t=1,n,m;
    while(scanf("%d%d",&a,&b)!=EOF&&a!=-1&&b!=-1)
    {
        max=-1; min=maxn;
        if(a==0&&b==0)
        {
			printf("Case %d is a tree.\n",t++);
			continue;
        }
		makeset(); cnt=0;
        cnt=0;
        while(a||b)
        {
            if(a<min) min=a;  if(b<min) min=b;
            if(a>max) max=a;  if(b>max) max=b;
            flag[a]=flag[b]=1;
            n=findfather(a);
            m=findfather(b);
            if(n==m)  cnt=1;
            else Union(n,m);
            scanf("%d%d",&a,&b);
        }
        if(cnt==1)  printf("Case %d is not a tree.\n",t++);
        else
        {
            for(i=min;i<=max;i++)
                if(flag[i]&&father[i]==i)
                {
                    cnt++;
                    if(cnt>1) break;
                }
			if(cnt==1) printf("Case %d is a tree.\n",t++);
			else printf("Case %d is not a tree.\n",t++);
        }
    }
    return 0;
}