Connect cities

input/code.cpp: In function 'int checkVisited(const std::vector<bool>&)':
input/code.cpp:8:19: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<bool>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
    8 |     for(int i=0; i< visited.size(); i++){
      |                  ~^~~~~~~~~~~~~~~~
input/code.cpp: In function 'void BFS(std::vector<bool>&, std::vector<std::vector<int> >&, const int&)':
input/code.cpp:22:23: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   22 |         for(int i=0; i<roads[x].size(); i++){
      |                      ~^~~~~~~~~~~~~~~~
input/code.cpp: In function 'int main()':
input/code.cpp:56:19: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   56 |     for(int i=1; i<caps.size(); i++){
      |...

#include <iostream>
#include <vector>
#include <queue>

using namespace std;

int checkVisited(const vector<bool>& visited){
    for(int i=0; i< visited.size(); i++){
        if(visited[i]==false) return i;
    }
    return -1;
}

void BFS(vector<bool>& visited, vector<vector<int>>& roads, const int& init){
    queue<int> q;
    q.push(init);

    while(not q.empty())
    {
        int x = q.front();
        q.pop();
        for(int i=0; i<roads[x].size(); i++){
            if(visited[roads[x][i]] == false) q.push(roads[x][i]);
        }
        visited[x] = true;
    }
}

int main(){
    int n,m;
    cin >> n >> m;

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

    for(int i=0; i<m; i++){
        int a,b;
        cin >> a >> b;
        roads[a-1].push_back(b-1);
        roads[b-1].push_back(a-1);
    }
    vector<int> caps;

    int last = checkVisited(visited);
    caps.push_back(last);

    while (last != -1)
    {
        BFS(visited, roads, last);
        last = checkVisited(visited);
        if(last != -1)caps.push_back(last);
    }

    cout << caps.size()-1 << endl;

    for(int i=1; i<caps.size(); i++){ 
        cout<<caps[0]+1<<" "<<caps[i]+1;
        cout << 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: TIME LIMIT EXCEEDED

Test 12

Verdict: ACCEPTED