hdu 1272 小希的迷宫 （简单并查集）

题目链接：http://acm.hdu.edu.cn/showproblem.php?pid=1272

给出一张图，问时候符合条件。

#pragma GCC optimize(2)
#include<stdio.h>
#include<algorithm>
#include<string.h>
#include<queue>
using namespace std;
const int maxn = 1e5 + 100;
const int inf = 0x3f3f3f3f;
typedef long long ll;
int f[maxn];
int vis[maxn];
bool circle;
int num;
int find(int x)
{
	if (x == f[x])
	{
		return x;
	}
	else
	{
		return f[x] = find(f[x]);
	}
}
void _union(int a, int b)
{
	if (a == b)
	{
		circle = true;
	}
	int x = find(a);
	int y = find(b);
	if (x != y)           //判环
	{
		f[x] = y;
		num++;           //引出一条边
	}
	else                  //如果a和b有共同的祖先，那么a和b和其祖先形成一个环
	{
		circle = true;
	}
}
int main()
{
	//freopen("C://input.txt", "r", stdin);
	while (1)
	{
		int a, b;
		num = 0;
		int vim = 0;
		circle = false;
		for (int i = 0; i <= maxn-1; i++)
		{
			f[i] = i;
			vis[i] = false;
		}
		scanf("%d%d", &a, &b);
		if (a == 0 && b == 0)
		{
			printf("Yes\n");
			continue;
		}
		if (a == -1 && b == -1)
		{
			break;
		}
		vis[a] = true;
		vis[b] = true;
		_union(a, b);
		while (1)
		{
			scanf("%d%d", &a, &b);
			if (a == 0 && b == 0)
			{
				break;
			}
			vis[a] = true;
			vis[b] = true;
			_union(a, b);
		}
		for (int i = 0; i <= maxn-1; i++)
		{
			if (vis[i])
			{
				vim++;          //点数
			}
		}
		if (!circle&&num + 1 == vim)
		{
			printf("Yes\n");
		}
		else
		{
			printf("No\n");
		}
	}
	return 0;
}