思路;
形成树的两个条件
1,图中不存在回路。
2.图是连通的。
#include<iostream>
#include<cstdio>
#include<cstring>
using namespace std;
int fa[100010],vis[100010];
int a,b,dian,bian,flag;
int find(int x){if(x!=fa[x]){fa[x]=find(fa[x]);}return fa[x];
}
void uni(int x,int y){if(!flag) return ;int fx=find(x);int fy=find(y);if(fx!=fy)fa[fx]=fy;
}
void mem(){for(int i=1;i<=100010;i++)fa[i]=i;bian=dian=0;flag=1;memset(vis,0,sizeof(vis));
}
void jud(){int te=0;for(int i=1;i<=100010;i++)if(vis[i]&&find(i)==i) te++;if(te==1||te==0) flag=1;else flag=0;
// cout<<"te="<<te<<endl;}
int main(){int ca=1;mem();while(scanf("%d%d",&a,&b)){if(a==-1&&b==-1) break;if(a==0&&b==0){if(flag) jud();if(flag) printf("Case %d is a tree.\n",ca++);else printf("Case %d is not a tree.\n",ca++);mem();}else{vis[a]=vis[b]=1; if(find(a)==find(b)){flag=0;}uni(a,b);}}return 0;
}