Connect cities

input/code.cpp: In function 'int main(int, const char**)':
input/code.cpp:30:24: warning: comparison of integer expressions of different signedness: 'size_t' {aka 'long unsigned int'} and 'int' [-Wsign-compare]
   30 |   for (size_t i = 0; i < roads; i++) {
      |                      ~~^~~~~~~
input/code.cpp:44:30: warning: comparison of integer expressions of different signedness: 'size_t' {aka 'long unsigned int'} and 'int' [-Wsign-compare]
   44 |   for (size_t city = 1; city < cities + 1; city++) {
      |                         ~~~~~^~~~~~~~~~~~

#include <iostream>
#include <string>
#include <vector>
using namespace std;

class Program {
  vector<vector<int>> adj;

  void main() {}
};

void dfs(int city, std::vector<int> *adj, std::vector<bool> &seen) {
  if (seen[city]) {
    return;
  }
  seen[city] = true;
  for (auto i : adj[city]) {
    dfs(i, adj, seen);
  }
}

int main(int argc, char const *argv[]) {
  std::string n, m;
  std::cin >> n >> m;
  int cities = std::stoi(n);
  int roads = std::stoi(m);

  std::vector<int> adj[cities + 1];

  for (size_t i = 0; i < roads; i++) {
    std::string as, bs;
    std::cin >> as >> bs;
    int a = std::stoi(as);
    int b = std::stoi(bs);

    adj[a].push_back(b);
    adj[b].push_back(a);
  }

  std::vector<bool> seen(cities + 1);

  std::vector<int> unique;

  for (size_t city = 1; city < cities + 1; city++) {
    if (!seen[city]) {
      unique.push_back(city);
    }
    dfs(city, adj, seen);
  }

  std::cout << unique.size() - 1 << std::endl;

  for (size_t i = 0; i < unique.size() - 1; i++) {
    std::cout << unique[i] << " " << unique[i + 1] << std::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