Connect cities

input/code.cpp: In function 'int main()':
input/code.cpp:44:27: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   44 |     for (int i = 0; i + 1 < clusters.size(); i++) {
      |                     ~~~~~~^~~~~~~~~~~~~~~~~

#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

int main() {
    ios::sync_with_stdio(0);
    cin.tie(nullptr);
    //freopen("input.txt", "r", stdin);

    int n, m;
    cin >> n >> m;
    vector<vector<int>> edges(n+1);
    for (int i = 0; i < m; i++) {
        int a, b;
        cin >> a >> b;
        edges[a].push_back(b);
        edges[b].push_back(a);
    }

    vector<int> stack;
    vector<bool> visited(n+1, false);
    vector<int> clusters;
    for (int i = 1; i <= n; i++) {
        if (!visited[i]) {
            clusters.push_back(i);
            visited[i] = true;
            stack.clear();
            stack.push_back(i);
            while (!stack.empty()) {
                int city = stack.back(); stack.pop_back();
                for (auto& v : edges[city]) {
                    if (!visited[v]) {
                        visited[v] = true;
                        stack.push_back(v);
                    }
                }
            }
        }
    }

    cout << clusters.size() - 1 << "\n";
    for (int i = 0; i + 1 < clusters.size(); i++) {
        cout << clusters[i] << " " << clusters[i + 1] << "\n";
    }
}

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