Is It A Tree?------HDOJ杭电1325（两种方法，可以用也可以不用并查集！！！！！！详解）

#include<stdio.h>
#define maxn 100000 + 6
int tree[maxn],bj[maxn];                      //数组e的元素值等于当前序号父级的序号，不一定是树的根
int find(int x)
{                                                            //找树根
    if(tree[x]==x)
        return x;
    else
        return find(tree[x]);
}

int main()
{
    int i,j,k;
    int t,n,m;
    int x,y,num;
    int w=0;
    while(scanf("%d%d",&m,&n)!=EOF)
    {
        if(m<0&&n<0)
            break;
        t=0;
        w++;
        if(!(m||n))
            printf("Case %d is a tree.\n",w);
        else
        {
            for(i=1;i<=maxn;i++)
            {
                tree[i]=i;
                bj[i]=0;
            }
            tree[n]=m;
            bj[m]=bj[n]=1;
            while(scanf("%d%d",&m,&n)!=EOF&&(m||n))
            {
                x=find(m);
                y=find(n);
                if(x==y)
                    t=1;                                             //判断是否是环
                else
                    tree[n]=m;                                       //即使存在元素更改了父级也不影响树的个数
                bj[m]=bj[n]=1;
            }
            if(t==1)
                printf("Case %d is not a tree.\n",w);
            else
            {
                num=0;
                for(i=1;i<=maxn;i++)
                {
                    if(bj[i]==1&&tree[i]==i)                      //统计树根个数，为1时才符合题意
                    {
                        num++;
                        if(num>1)
                            break;
                    }
                }
                if(num>1)
                    printf("Case %d is not a tree.\n",w);
                else
                    printf("Case %d is a tree.\n",w);
            }
        }
    }
    return 0;
}

#include "stdio.h"
#include<iostream>
#define m 100001
using namespace std;
int main()
{
    int i,j,k;
    int a,b,flag;
    int tree[m];
    int v[m];
    int ans=1,flag2;
    while(cin>>a>>b)
    {
        if(a<0&&b<0)
            break;
        flag=0;
        flag2=0;
        memset(tree,0,m-1);
        memset(v,0,m-1);
        int e=0;
        while(a&&b)
        {
            flag2=1;
            if(!v[a])
            {
                v[a]=1;
                e++;
            }
            if(!v[b])
            {
                v[b]=1;
                e++;
            }
            e--;
            if(tree[b]==0)//b is root
                tree[b]=1;
            else
            {
                flag=1;
                break;
            }
            cin>>a>>b;
        }
        while(a&&b)
        {
            scanf("%d %d",&a,&b);
        }
        if((!flag&&e==1)||flag2==0)
            printf("Case %d is a tree.\n",ans);
        else
            printf("Case %d is not a tree.\n",ans);
        ans++;
    }
    return 0;
}