Connect cities

input/code.cpp: In constructor 'BFS::BFS(int)':
input/code.cpp:10:7: warning: 'BFS::n' will be initialized after [-Wreorder]
   10 |   int n;
      |       ^
input/code.cpp:9:20: warning:   'std::vector<int> BFS::pred' [-Wreorder]
    9 |   std::vector<int> pred;
      |                    ^~~~
input/code.cpp:11:3: warning:   when initialized here [-Wreorder]
   11 |   BFS(int n): nodes(n), color(n, 0), n(n), pred(n, -1){}
      |   ^~~
input/code.cpp: In function 'int main()':
input/code.cpp:58:18: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   58 |   for (int i=0; i<sepReps.size()-1; ++i){
      |                 ~^~~~~~~~~~~~~~~~~

#include <queue>
#include <vector>
struct BFS{
  //nodes[i] = neighbor nodes of i
  std::vector<std::vector<int>> nodes;
  std::vector<int> color; // 0 = unvisited, 1 = custom, 2 = custom
  std::queue<int> q;
  // std::vector<int> dist;
  std::vector<int> pred;
  int n;
  BFS(int n): nodes(n), color(n, 0), n(n), pred(n, -1){}
  void addNeighbor(int i, int nb){
    nodes[i].push_back(nb);
  }
  void traverse(){
    for (int i = 0; i<n; ++i){
      if (color[i]!=0) continue;
      color[i] = 1;
      q.push(i);
      while (!q.empty()) {
        int u = q.front(); 
        q.pop();
        for (int nb : nodes[u]){
          if (color[nb] == 0){
            pred[nb] = i;
            color[nb] = 1;
            q.push(nb);
          }
        }
      }
    }
  }
};


#include <iostream>
int main(){
  int n, m;
  std::cin >> n >> m;
  BFS bfs(n);
  for (int road = 0; road<m; ++road){
    int city1, city2;
    std::cin >> city1 >> city2;
    bfs.addNeighbor(city1-1, city2-1);
    bfs.addNeighbor(city2-1, city1-1);
  }

  bfs.traverse();
  std::vector<int> sepReps;
  for (int city = 0; city<n; ++city){
    if (bfs.pred[city] == -1){
      sepReps.push_back(city);
    }
  }

  std::cout << sepReps.size() - 1<< "\n";

  for (int i=0; i<sepReps.size()-1; ++i){
    std::cout << sepReps[i] + 1 << " " << sepReps[i+1] + 1 << "\n";
  }
  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