hdu 1272

***题意就是判断图是否连通和是否有回路

思路：判断连通性可以看整张图有几个根节点，回路的判断可以看输入的两个节点是否是同一根节点，若是就有回路***

#include<iostream>
#include<cstdio>
#include<cstring>
#include<cstdlib>
#include<cmath>
#include<cctype>
#include<queue>
#include<vector>
#include<algorithm>

using namespace std;
typedef long long LL;
#define N 1001000
#define INF 0x3f3f3f

int father[N], vis[N], flag;

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


int main()
{
    int a, b;

    while(scanf("%d%d", &a, &b), a!=-1 || b!=-1)
    {
        if(a==0&&b==0)
        {
            printf("Yes
");
            continue;
        }
        for(int i=1; i<N; i++)
            father[i]=i;
        memset(vis, 0, sizeof(vis));
        flag=1;
        vis[a]=vis[b]=1;
        int x=Find(a);
        int y=Find(b);
        if(x!=y)
            father[x]=y;
        else
            flag=0;

        while(scanf("%d%d", &a, &b), a || b)
        {
            vis[a]=vis[b]=1;
            int x=Find(a);
            int y=Find(b);
            if(x!=y)
                father[x]=y;
            else
                flag=0;
        }

        int k=0;
        if(flag)
        {
            for(int i=1; i<N; i++)
            {
                if(vis[i]&&father[i]==i)
                    k++;
                if(k>1)
                {
                    flag=0;
                    break;
                }
            }
        }
        if(flag)
            printf("Yes
");
        else
            printf("No
");
    }
    return 0;
}