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:58:3: note: in expansion of macro 'REP'
   58 |   REP(i, 0, comps.size() - 1) {
      |   ^~~

#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;
typedef unordered_set<int> usi;

void dfs(int s, vi adj[], bool vis[], vi& comp) {
  if (vis[s]) {
    return;
  }
  vis[s] = true;
  comp.push_back(s);
  // process s
  for (auto u : adj[s]) {
    dfs(u, adj, vis, comp);
  }
}

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

  vi adj[n + 1];

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

  // determine connected components

  bool vis[n + 1] = {};
  vector<vi> comps;

  REP(i, 1, n + 1) {
    if (vis[i]) continue;
    vi comp;
    dfs(i, adj, vis, comp);
    comps.push_back(comp);
  }

  // connect disjoint connected components
  cout << comps.size() - 1 << BR;
  REP(i, 0, comps.size() - 1) {
    cout << comps[i].back() << " " << comps[i + 1].front() << BR;
  }

  return EXIT_SUCCESS;
}

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