Connect cities

input/code.cpp: In function 'int main()':
input/code.cpp:5:40: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<std::vector<int> >::size_type' {aka 'long unsigned int'} [-Wsign-compare]
    5 | #define REP(i, a, b) for (int i = a; i < b; ++i)
      |                                        ^
input/code.cpp:62:3: note: in expansion of macro 'REP'
   62 |   REP(i, 1, connected_components.size()) {
      |   ^~~

#define F first
#define S second
#define PB push_back
#define MP make_pair
#define REP(i, a, b) for (int i = a; i < b; ++i)
#define BR "\n"

#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef vector<int> vi;
typedef vector<bool> vb;
typedef pair<int, int> pi;

unordered_set<int> not_visited;
vector<vi> adj;
vector<vi> connected_components;

void dfs(int s, int ci) {
  if (not_visited.count(s) == 0) return;
  connected_components[ci].push_back(s);
  not_visited.erase(s);
  // process s
  for (auto u : adj[s]) {
    dfs(u, ci);
  }
}

int main() {
  // freopen("cities.txt", "r", stdin);
  int n, m;
  cin >> n >> m;

  adj = vector<vi>(n + 1);

  REP(i, 0, m) {
    int a, b;
    cin >> a >> b;
    adj[a].push_back(b);
    adj[b].push_back(a);
    not_visited.insert(a);
    not_visited.insert(b);
  }

  // determine connected components

  int ci = 0;
  while (not_visited.size() > 0) {
    connected_components.push_back(vi());
    dfs(*not_visited.begin(), ci);
    ci++;
  }

  vector<pi> edges_to_add;

  // connect disjoint connected components
  cout << ci - 1 << BR;

  auto first = connected_components[0];
  REP(i, 1, connected_components.size()) {
    cout << first[0] << " " << connected_components[i][0] << BR;
  }

  return EXIT_SUCCESS;
}

Test 1

Verdict: WRONG ANSWER

Test 2

Verdict: WRONG ANSWER

Test 3

Verdict: ACCEPTED

Test 4

Verdict: WRONG ANSWER

Test 5

Verdict: ACCEPTED

Test 6

Verdict: WRONG ANSWER

Test 7

Verdict: WRONG ANSWER

Test 8

Verdict: WRONG ANSWER

Test 9

Verdict: WRONG ANSWER

Test 10

Verdict: WRONG ANSWER

Test 11

Verdict: WRONG ANSWER

Test 12

Verdict: WRONG ANSWER