Connect cities

input/code.cpp: In function 'int main()':
input/code.cpp:43:23: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   43 |     for (int i = 1; i < r.size(); i++) {
      |                     ~~^~~~~~~~~~

#include <bits/stdc++.h>

using namespace std;

void dfs(int s, vector<vector<int>> &adj, vector<bool> &visited,
         vector<int> &res) {
    if (visited[s])
        return;
    visited[s] = true;

    res.push_back(s);

    for (auto u : adj[s]) {
        dfs(u, adj, visited, res);
    }
}

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

    vector<vector<int>> adj(n + 1);

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

    vector<bool> visited(adj.size(), false);
    vector<int> r;

    for (int i = 1; i <= n; i++) {
        if (!visited[i]) {
            vector<int> c;
            dfs(i, adj, visited, c);
            r.push_back(i);
        }
    }

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