题解 | #连通图#

#include <iostream>
using namespace std;


const int N = 100010;
int father[N];
int height[N];

void initial(int n) {
    for (int i = 1; i <= n; i++) {
        father[i] = i;
        height[i] = 0;
    }
    return;
}

int find(int n) {
    if (father[n] != n) father[n] = find(father[n]);
    return father[n];
}

void Union(int a, int b) {
    a = find(a);
    b = find(b);
    if (a != b) {
        if (height[a] < height[b]) father[a] = b;
        else if (height[a] > height[b]) father[b] = a;
        else {
            father[a] = b;
            height[b]++;
        }
    }
}

int main() {
    int n, m;
    while (cin >> n >> m) {
        if (n == 0) break;
        initial(n);

        while (m--) {
            int a, b;
            cin >> a >> b;
            Union(a, b);
        }
        int ans = 0;
        for (int i = 1; i <= n; i++) {
            if (find(i) == i) ans++;
        }
        if(ans==1) cout<<"YES"<<endl;
        else cout<<"NO"<<endl;
    }
}
// 64 位输出请用 printf("%lld")