Connect cities

input/code.cpp: In function 'int main()':
input/code.cpp:43:21: 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 = 0; i < group_roots.size() - 1; i++) {
      |                   ~~^~~~~~~~~~~~~~~~~~~~~~~~

#include <iostream>
#include <vector>


int main() {
  int n_cities, n_roads;
  std::cin >> n_cities >> n_roads;
  std::vector<std::vector<int>> adjacencies(n_cities);
  std::vector<bool> discovered(n_cities, false);
  int first_undiscovered = 0;
  for (int i = 0; i < n_roads; i++) {
    int a, b;
    std::cin >> a >> b;
    a--;
    b--;
    adjacencies[a].push_back(b);
    adjacencies[b].push_back(a);
  }
  std::vector<int> group_roots;
  std::vector<int> stack;
  while (first_undiscovered < n_cities) {
    // iterative DFS over cities
    group_roots.push_back(first_undiscovered + 1);
    stack.push_back(first_undiscovered);
    while (!stack.empty()) {
      int v = stack.back();
      stack.pop_back();
      if (discovered[v]) {
        continue;
      }
      discovered[v] = true;
      std::copy(adjacencies[v].begin(), adjacencies[v].end(),
                std::back_inserter(stack));
    }
    // Scan through discovery list to find new beginning
    while (discovered[first_undiscovered]) {
      first_undiscovered++;
    }
  }

  // k is number of groups - 1
  std::cout << group_roots.size() - 1 << std::endl;
  for (int i = 0; i < group_roots.size() - 1; i++) {
    std::cout << group_roots[i] << " " << group_roots[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