（并查集）Is It A Tree? --POJ--1308

链接：

http://poj.org/problem?id=1308

http://acm.hust.edu.cn/vjudge/contest/view.action?cid=82830#problem/N

代码：

#include<stdio.h>
#include<string.h>
#include<stdlib.h>
#include<iostream>
#include<algorithm>
using namespace std;

#define N 100005

int f[N];
bool r[N];

void IN()
{
    memset(r, false, sizeof(r));
    for(int i=0; i<N; i++)
        f[i]=i;
}

int Find(int x)
{
    while(x!=f[x])
    {
        x=f[x];
    }
    return f[x];
}

int main()
{
    int a, b, fa, fb, i,sum=0, flag1=0, flag2=0, k=1, MIN, MAX;

    IN();
    while(scanf("%d%d", &a, &b), a>=0 || b>=0)
    {
        if(a==0 && b==0)
        {
          if(sum==0)
          {
               printf("Case %d is a tree.
", k++);
               continue;
          }
          int z = Find(MIN);
          for(i=MIN; i<=MAX; i++)
          {
              if(r[i])
              {
                  if(z!=Find(i))  // 根（父）节点不同
                  {
                      flag2=1;
                      break;
                  }
              }
          }

          if(flag1 || flag2)
            printf("Case %d is not a tree.
", k++);
          else
            printf("Case %d is a tree.
", k++);
          IN();
          sum=0, flag1=0, flag2=0;
        }
        else
        {
           sum++;
           r[a] = 1, r[b] = 1;
           MIN = min(MIN, min(a ,b)), MAX = (MAX, max(a, b));

           fa = Find(a), fb = Find(b);
           if(fa != fb)
            f[fa] = fb;
           else      //构成了环
            flag1=1;
        }

    }
    return 0;
}