Connect cities

input/code.cpp: In function 'int main()':
input/code.cpp:47:23: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   47 |     for (int i = 0; i < roads.size() - 1; i++) {
      |                     ~~^~~~~~~~~~~~~~~~~~

#include <iostream>
#include <vector>
#include <queue>

using namespace std;

void bfs(int start, vector<bool> &visited, vector<vector<int>> &cities, queue<int> &queue) {
    visited[start] = true;
    queue.push(start);
    while (!queue.empty()) {
        int s = queue.front(); queue.pop();
        // process node s
        for (auto u : cities[s]) {
            if (visited[u]) continue;
            visited[u] = true;
            queue.push(u);
        }
    }
}

int main() {
    int n, m;
    cin >> n >> m;

    vector<vector<int>> cities(n);
    vector<int> roads;

    for (int i = 0; i < m; i++) {
        int r1, r2;
        cin >> r1 >> r2;
        cities[r1-1].push_back(r2-1);
        cities[r2-1].push_back(r1-1);
    }
    
    vector<bool> visited(n, false);
    queue<int> q;

    for (int i = 0; i < n; i++) {
        if (!visited[i]) {
            roads.push_back(i+1);
            bfs(i, visited, cities, q);
        }
    }

    cout << roads.size() - 1 << endl;

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

    return 0;
}

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