北大OJ1308

#include<iostream> #include <cstdio> #include<cstring> using namespace std; struct node {     int u,v; }edge[1005]; int parent[1005];   int findset(int x) {     if(parent[x]==-1)         return x;     parent[x]=findset(parent[x]);     return parent[x]; }   void Union(int x,int y) {     parent[y]=x; }   int main() {     int a,b,i=0,flag,u,v,t,k=1;     while(1)     {         flag=1;i=1;         cin>>a>>b;         if(a==-1&&b==-1) break;         memset(parent,-1,sizeof(parent));           if(a==0&&b==0)         {             cout<<"Case "<<k++<<" is a tree."<<endl;             continue;         }         edge[0].u=a;         edge[0].v=b;         while(cin>>a>>b&&a&&b)         {             edge[i].u=a;             edge[i++].v=b;         }           for (t=0;t<i;t++)         {             u= findset(edge[t].u);             v= findset(edge[t].v);             if(u==v||v!=edge[t].v)  /* 成环或者一个节点的入度大于 1 */            {                 flag=0;                 break;             }               else Union(u,v);//合并顶点         }         if(flag)         {             u=findset(edge[0].u);             for(t=1;t<i&&u==findset(edge[t].u);t++);             if(t>=i) cout<<"Case "<<k<<" is a tree."<<endl;             else cout<<"Case "<<k<<" is not a tree."<<endl;         }         else cout<<"Case "<<k<<" is not a tree."<<endl;             k++;     }     return 0; }