Connect cities

#include <bits/stdc++.h>

using namespace std;

int parent(int a, vector<int>& uf) {
    if (uf[a] < 0) return a;
    int p = parent(uf[a], uf);
    uf[a] = p;
    return uf[a];
}

void join(int a, int b, vector<int>& uf) {
    int ra = parent(a, uf);
    int rb = parent(b, uf);
    if (ra != rb) uf[rb] = ra;
}

int main() {

    int n, m;
    cin >> n >> m;

    vector<int> uf(n);
    for (int i = 0; i < n; i++) uf[i] = -1;

    while (m--) {
        int a, b;
        cin >> a >> b;
        a--;
        b--;
        join(a, b, uf);
    }


    vector<pair<int, int>> vec;
    for (int i = 1; i < n; i++) {
        if (parent(i, uf) != parent(0, uf)) {
            // cout << 1 << " " << i+1 << endl;
            vec.push_back(make_pair(0, i));
            join(i, 0, uf);
        }
    }

    cout << vec.size() << endl;
    for (auto &[a, b] : vec) cout << (a+1) << " " << (b+1) << endl;

}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED