#include<iostream>
#include<cstdio>
#include<cstring>
#include<string>
#include<algorithm>
using namespace std;
int f[1005],tree[1005];
int main()
{
    int a,b,ca(1),k,i;
    while(scanf("%d%d",&a,&b),a!=-1||b!=-1)
    {
        int flag=0;
        memset(f,0,sizeof(f));
        memset(tree,0,sizeof(tree));
        if(a==0&&b==0) { printf("Case %d is a tree.\n",ca++);continue;}
        if(a==b) flag=1;
        f[b]=a;
        tree[a]=1,tree[b]=1;
        int c,d;
        while(scanf("%d%d",&c,&d),c||d)
        {
            if(f[d]!=0)flag=1;
            f[d]=c;
            tree[c]=1,tree[d]=1;
        }

        if(flag){printf("Case %d is not a tree.\n",ca++);continue;}
        int ans=0;
        for(i=0;i<1005;i++)
        if(f[i]==0&&tree[i]==1)ans++;
        if(ans==1)printf("Case %d is a tree.\n",ca++);
        else printf("Case %d is not a tree.\n",ca++);
    }
 return 0;
}
这种方法好多了，一开始想多了，不知道卡在哪里了，总之A了。。。