Непонятная дичь

#include <iostream>
#include <vector>
using namespace std;
#define N 50000
vector<int> a[N];
int users[N];
int n;
 void dfs(int v, int countt) {
    users[v] = countt;
    for (int j = 1; j <= a[v].size() ; j++) {
        if (users[j] == 0) {
            users[j] = countt;
            dfs(j, countt);
        }

    }
}
int main() {
    int n, m;
    cin >> n >> m;
    int countt = 1;
    for (int i = 0; i < m; i++) {
        int a1, a2;
        cin >> a1 >> a2;
        a[a1].push_back(a2);
        a[a2].push_back(a1);
    }
    for (int i = 0; i < n; i++){
        if (users[i] == 0) dfs(i, countt);
    countt++;
    }
    cout << countt << "\n";
    int k = 1;
    for (int i = 0; i < countt; i++) {
        for (int j = 0; i < n; i++) {
            if (users[j] == k) cout << j << " ";
        }
        cout << endl;
        k++;
    }
    return 0;
}