Connect cities

input/code.cpp: In function 'int main()':
input/code.cpp:32:22: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   32 |     for(int c = 1; c < ne.size(); c++) {
      |                    ~~^~~~~~~~~~~

#include <bits/stdc++.h>
using namespace std;

int main (void) {
    int n, m;
    cin >> n >> m;
    vector<vector<int>> roads(n+1);
    for (int i = 0; i < m; i++){
        int c, c2;
        cin >> c >> c2;
        roads[c].push_back(c2);
        roads[c2].push_back(c);
    }

    vector<int> v(n+1);
    vector<int> ne;
    
    stack<int> s;
    for(int i = 1; i <= n; i++) {
        s.push(i);
        while(!s.empty()) {
            auto c = s.top(); s.pop();
            if(!v[c]) {
                v[c] = 1;
                if(i == c) ne.push_back(c);
                for(const auto& r : roads[c]) s.push(r);
            }
        }
    }
    int count = max(0, (int)ne.size()-1);
    cout << count << '\n';
    for(int c = 1; c < ne.size(); c++) {
        cout << ne[c] << ' ' << ne[c-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