hdu1272 小希的迷宫 基础并查集

#include <iostream>
#include <cstdlib>
#include <cstdio>
#include <algorithm>
using namespace std;

const int M = 100005;
int a, b;
int father[M];       //记录父节点 
bool circle;         //判断是否存在环 
bool visit[M];       //用来记录顶点数  
int edgenum, vnum;    //分别表示边数，顶点数 

void initial()
{
    for (int i = 0; i<M; i++)
        father[i] = i, visit[i] = false;
    circle = false;
    edgenum = vnum = 0;
}

//(查)
int find(int x)
{
    return x == father[x] ? x : father[x] = find(father[x]);     //找祖先节点 + 路径压缩 
}

//(并)
void merge(int a, int b)
{
    if (a == b)
        circle = true;
    int x, y;
    x = find(a);
    y = find(b);
    if (x != y)
    {
        father[x] = y;
        edgenum++;       //引出一条边 
    }
    else
        circle = true;   //x==y，说明他们是同一个祖先，一旦连通便与祖先3者成环 
}

int main()
{
    while (true)
    {
        initial();
        scanf("%d%d", &a, &b);
        if (a == 0 && b == 0)
        {     //为空树 
            printf("Yes
");
            continue;
        }
        if (a == -1 && b == -1)
            break;
        visit[a] = true;
        visit[b] = true;
        merge(a, b);
        while (true)
        {
            scanf("%d%d", &a, &b);
            if (a == 0 && b == 0)
                break;
            visit[a] = true;
            visit[b] = true;
            merge(a, b);
        }
        for (int i = 0; i < M; i++)
        {
            if (visit[i])
                vnum++;
        }
        if (!circle && edgenum + 1 == vnum)
            printf("Yes
");
        else
            printf("No
");
    }
    return 0;
}