poj 1308 （并查集）

#include<cstdio>
#include<iostream>
using namespace std;
#define MAXN 1000
int father[MAXN];
int rank[MAXN];
int a[MAXN],b[MAXN];
void make_set(int x)
{
    father[x]=x;
    rank[x]=0;
}
int find_set(int x)
{
    if(x==father[x]) return x;
    return father[x]=find_set(father[x]);
}
void union_set(int x,int y)
{
    int rx=find_set(x);
    int ry=find_set(y);
    father[ry]=rx;

}
int main()
{
    int CASE=0;
    int ra,rb,n;
    while(scanf("%d%d",&a[1],&b[1])!=EOF)
    {
        if(a[1]==-1&&b[1]==-1) return 0;
        if(a[1]==0&&b[1]==0) 
        {
            printf("Case %d is a tree.\n",++CASE);
            continue;
        }
        int i=1;
        while(1)
        {
            i++;
            scanf("%d%d",&a[i],&b[i]);
            if(a[i]==0&&b[i]==0) break;
        }
        n=i;
        int ok=1;
        for(i=1;i<MAXN;i++)
            make_set(i);
        for(i=1;i<n;i++)
        {
            ra=find_set(a[i]);
            rb=find_set(b[i]);
            if(ra==rb) 
            {
                ok=0;
                break;
            }            
            else 
                union_set(a[i],b[i]);
        }
        int s=find_set(a[1]);
        for(i=1;i<n;i++)
        {
            ra=find_set(a[i]);
            rb=find_set(b[i]);
            if(s!=ra||s!=rb) 
            {
                ok=0;
                break;
            }
            
        }
        if(ok==1) printf("Case %d is a tree.\n",++CASE);
        else printf("Case %d is not a tree.\n",++CASE);
    }
}