Connect cities

input/code.cpp: In function 'int main()':
input/code.cpp:73:13: warning: variable 'm' set but not used [-Wunused-but-set-variable]
   73 |   for (auto m : roads) {
      |             ^

#include <bits/stdc++.h>

using namespace std;
typedef long long ll;
typedef vector<int> vi;
typedef pair<int, int> pi;
typedef set<int> si;
typedef set<set<int>> ssi;
typedef vector<pair<int, int>> vpi;
#define F first
#define S second
#define PB push_back
#define MP make_pair

#if DO_DBG
#define DBG(x) cerr << x << "\n";
#define DBG2(x) cerr << x;
#else
#define DBG(x)
#define DBG2(x)
#endif

#define loop(n) for (int i = 0; i < n; i++)

class UnionFind {

public:
  vector<int> parent;
  vector<int> rank;
  UnionFind(int size) {
    parent.resize(size);
    for (int i = 0; i < size; i++) {
      parent[i] = i;
    }
    rank.resize(size, 0);
  }
  int find(int i) {
    if (parent[i] == i) {
      return i;
    }
    return find(parent[i]);
  }
  void unite(int i, int j) {
    int irep = find(i);
    int jrep = find(j);
    if (irep == jrep)
      return;
    if (rank[irep] < rank[jrep]) {
      parent[irep] = jrep;
    } else if (rank[irep] > rank[jrep]) {
      parent[jrep] = irep;
    } else {
      parent[jrep] = irep;
      rank[irep]++;
    }
  }
};

int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  DBG("INIT")
  int n, m;
  cin >> n >> m;
  vpi roads;
  loop(m) {
    int src, dst;
    cin >> src >> dst;
    roads.push_back(MP(src - 1, dst - 1));
  }
  DBG("n: " << n)
  DBG("m: " << m)
  for (auto m : roads) {
    DBG(m.F << " -> " << m.S)
  }
  UnionFind islands{n};

  for (auto m : roads) {
    auto i1res = islands.find(m.F);
    auto i2res = islands.find(m.S);
    bool sameSet = i1res == i2res;
    if (!sameSet)
      islands.unite(m.F, m.S);
  }
  vi newRoads;
  for (int i = 1; i < n; i++) {
    if (islands.find(i) != islands.find(0)) {
      islands.unite(i, 0);
      newRoads.push_back(i);
    }
  }
  cout << newRoads.size() << "\n";
  for (auto a : newRoads) {
    cout << "1 " << a + 1 << "\n";
  }
}

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