POJ1308 Is It A Tree?

#include<cstdio>
#include<cstdlib>
#include<cmath>
#include<iostream>
#include<algorithm>
#include<cstring>
#include<vector>
#include<queue>
#define INF 0x3f3f3f3f
#define MAX 1000005

using namespace std;

int father[MAX],vis[MAX];

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

int main()
{
    int a,b,ok,i,k,cnt=1;

    while(scanf("%d%d",&a,&b),a!=-1 || b!=-1)
    {
        k=0;
        memset(vis,0,sizeof(vis));

        vis[a]=1;
        vis[b]=1;

        ok=1;

        if(!a && !b)
        {
            printf("Case %d is a tree.\n",cnt++);
            continue;
        }

        for(i=0; i<MAX; i++)
            father[i]=i;

        int x=Find(a);
        int y=Find(b);

        if(x!=y)
        {
            father[x]=y;
        }

        else
            ok=0;

        while(scanf("%d%d",&a,&b),a||b)
        {
            vis[a]=1;
            vis[b]=1;
            int x=Find(a);
            int y=Find(b);

            if(x!=y)
            {
                father[x]=y;
            }
            else
                ok=0;//标记一下
        }

        if(ok)//当ok=0时就不进入了，节省下时间
        {
            for(i=0;i<MAX;i++)
            {
                if(vis[i] && father[i]==i)
                    k++;
            }
        }

        if(k>1)
            ok=0;

        if(ok)
            printf("Case %d is a tree.\n",cnt++);
        else
            printf("Case %d is not a tree.\n",cnt++);
    }

    return 0;
}