Connect cities

input/code.cpp: In function 'int main()':
input/code.cpp:43:20: 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 < cities.size()-1; i++){
      |                  ~~^~~~~~~~~~~~~~~~~

#include <iostream>
#include <queue>

using namespace std;

void dfs(vector<vector<int>>&v, vector<bool> &visited, int city){
  if(visited[city]) return;
  else{
    visited[city] = true;
    for(int u : v[city]){
      dfs(v, visited, u);
    }
  }
}

int main(){
  int n, m, x, y;

  cin >> n >> m;

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

  for(int i = 0; i < m; i++){
    cin >> x >> y;
    v[x].push_back(y);
    v[y].push_back(x);
  }

  vector<bool> visited(n, false);
  vector<int> cities;
  int city;

  for(int i = 1; i < n+1; i++){
    city = i;
    if(not visited[i]){
      dfs(v, visited, city);
      cities.push_back(city);
    }
  }

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

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