吉林大学机试 连通图 Easy

#include<iostream>
#include<string>
#include<vector>
#include<map>
using namespace std;

const int maxn = 1020;

int father[maxn];

int m, n;
bool ma[maxn][maxn];

void init(int n) {
    for (int i = 0; i <= n; i++)
        father[i] = i;
}

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

void unfather(int a, int b) {
    int aa = findfather(a);
    int bb = findfather(b);
    father[aa] = bb;
}

int cntnum(int n) {
    int cnt = 0;
    for (int i = 1; i <= n; i++) {
        if (father[i] == i)
            cnt++;
    }
    return cnt;
}

int main() {
    while (cin >> n >> m) {
        if(n==0)
            continue;
        fill(ma[0], ma[0] + maxn * maxn, false);
        init(n);
        int a, b;
        for (int i = 0; i < m; i++) {
            cin >> a >> b;
            if (!ma[a][b]) {
                ma[a][b] = ma[b][a] = true;
                unfather(a, b);
            }
        }
        if (cntnum(n) != 1)
            cout << "NO" << endl;
        else
            cout << "YES" << endl;
    }
    return 0;
}