题解 | #连通图#

//并查集没什么好说的
#include<iostream>
#include<set>

using namespace std;


int father[1001];
void initialUnion()
{
	for (int i = 0; i < 1001; i++)
	{
		father[i] = i;
	}
}
int findFather(int x)
{
	if (x == father[x]) return x;
	else
	{
		father[x] = findFather(father[x]);
		return father[x];
	}
}
void unionFather(int x, int y)
{
	int x_father = findFather(x);
	int y_father = findFather(y);
	father[y_father] = father[x_father];
}

int main()
{
	int N, M;
	while (scanf("%d %d",&N,&M)!=EOF)
	{
		if (N == 0 && M == 0)
		{
			break;
		}
		initialUnion();
		for(int i=0;i<M;i++)
		{
			int x, y;
			scanf("%d %d", &x, &y);
			unionFather(x, y);
		}
		set<int>blocks;
		for (int i = 1; i <= N; i++)
		{
			blocks.insert(findFather(i));

		}
		if (blocks.size() == 1)
		{
			cout << "YES" << endl;
		}
		else
		{
			cout << "NO" << endl;
		}

	}

}