POJ-1308-Is It A Tree?

#include<cstdio>
#include<cstring>
#include<iostream>
using namespace std;
const int maxn=10011;
int p[maxn];
bool vis[maxn];
int find(int x)
{
if(p[x]==-1)
return x;
return p[x]=find(p[x]);
}
void Union(int x,int y)
{
p[min(x,y)]=max(x,y);
}
int main()
{
int a,b,cas=1;
while(scanf("%d%d",&a,&b)!=EOF)
{
if(a==-1&&b==-1)
break;
if(a==0&&b==0)
{
printf("Case %d is a tree.\n",cas++);
continue;
}
memset(p,-1,sizeof(p));
memset(vis,0,sizeof(vis));
int n=0,ans=0;
n=max(a,max(n,b));
int x=find(a);
int y=find(b);
vis[a]=vis[b]=1;
if(x==y)
ans++;
else
Union(x,y);
while(scanf("%d%d",&a,&b)&&(a+b))
{
n=max(a,max(n,b));
vis[a]=vis[b]=1;
x=find(a);
y=find(b);
if(x==y)
ans++;
else
Union(x,y);
}
for(int i=1;i<=n;i++)
if(vis[i]&&p[i]==-1)
ans++;
if(ans==1)
printf("Case %d is a tree.\n",cas++);
else
printf("Case %d is not a tree.\n",cas++);
}
return 0;
}