Connect cities

input/code.cpp: In function 'int main(int, char**)':
input/code.cpp:45:27: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   45 |     for (int i = 0; i + 1 < starts.size(); i++) {
      |                     ~~~~~~^~~~~~~~~~~~~~~

#include <iostream>
#include <map>
#include <unistd.h>
#include <vector>


int main(int argc, char** argv) {
    int n, m;
    std::cin >> n >> m;

    std::vector<std::vector<int>> neightbours(n + 1);

    for (int x=0; x<m; x++) {
        int a, b;
        std::cin >> a >> b;
        neightbours[a].push_back(b);
        neightbours[b].push_back(a);
    }

    std::vector<bool> visited(n + 1, false);

    std::vector<int> starts;
    
    for (int i=1; i<=n; i++) {
        if (visited[i]) continue;

        starts.push_back(i);
        visited[i] = true;
        std::vector<int> to_visit = {i};

        while (!to_visit.empty()) {
            int city = to_visit.back();
            to_visit.pop_back();

            for (int neightbour : neightbours[city]) {
                if (!visited[neightbour]) {
                    visited[neightbour] = true;
                    to_visit.push_back(neightbour);
                }
            }
        }
    }
    
    std::cout << starts.size() -1 << std::endl;
    for (int i = 0; i + 1 < starts.size(); i++) {
        std::cout << starts[i] << " " << starts[i+1] << std::endl;
    }
    

    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