HDU1325

并查集方法
#include<cstdio>
#include<cstdlib>
#include<iostream>
#include<map>
#include<algorithm>
#include<memory.h> 
using namespace std;
int fa[100010],ru[100010];
int s[100010],cnt,num;
int _fa(int v)
{
    if(v==fa[v]) return v;
    fa[v]=_fa(fa[v]);
    return fa[v];
}
int main()
{
    int a,b;
    int faa,fab; 
    int Case=0;
    bool flag=true;
    while(1){
        flag=true;//更新 
        memset(fa,0,sizeof(fa));
        memset(ru,0,sizeof(ru));
        cnt=0;
        while(scanf("%d%d",&a,&b)&&a&&b){
            if(a<0||b<0) return 0;
             if(fa[a]==0) {fa[a]=a;s[++cnt]=a;}//离散化 
             if(fa[b]==0) {fa[b]=b;s[++cnt]=b;}
             faa=_fa(a);
             fab=_fa(b);
             ru[b]++;
             if(ru[b]>1)  flag=false;
             if(faa==fab) flag=false;
             fa[fab]=faa;
           }            
            faa=_fa(s[1]);//判断是否联通 
            for(int i=2;i<=cnt;i++){
                fab=_fa(s[i]);
                if(faa!=fab){
                    flag=false;
                    break;
                }
            }
            if(flag) printf("Case %d is a tree.
",++Case);
            else printf("Case %d is not a tree.
",++Case);
    }
    return 0;
}

边顶关系
#include<iostream>
#include<cstdio>
#include<cstring>
using namespace std;
int f[100005];
int main()
{
    int a,b,flag,i,j;
    int t=1;
    while(1)
    {
        j=0;
        i=0;
        flag=0;
        memset(f,0,sizeof(f));
        while(scanf("%d%d",&a,&b)&&a&&b)
        {
             if(a<0||b<0)
                return 0;
            if(f[b]-1==1)
                flag=1;
            if(f[a]==0)
                j++;
            if(f[b]==0)
                j++;
            f[a]=1;f[b]=2;i++;
        }
        if(flag==0&&j==i+1)
            printf("Case %d is a tree.
",t++);
            else printf("Case %d is not a tree.
",t++);
    }
}