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;
}

4000
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;
}