Connect cities

input/code.cpp: In function 'int main()':
input/code.cpp:45:23: 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 = 1; i < elem.size(); i++) {
      |                     ~~^~~~~~~~~~~~~

#include <bits/stdc++.h>

int main() {
    std::ios_base::sync_with_stdio(false);
    std::cin.tie(nullptr);

    int n, m;
    std::cin >> n >> m;

    std::vector<std::vector<int>> graph(n+1);
    for (int i = 0; i < m; i++) {
        int a,b;
        std::cin >> a >> b;
        graph[a].push_back(b);
        graph[b].push_back(a);
    }

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

    for (int i = 1; i <= n; i++) {
        if (visited[i]) {
            continue;
        }
        visited[i]=true;

        elem.push_back(i);

        std::stack<int> stack;
        stack.push(i);

        while (!stack.empty()) {
            const int v = stack.top();
            stack.pop();
            for (int w : graph[v]) {
                if (!visited[w]) {
                    visited[w] = true;
                    stack.push(w);
                }
            }
        }
    }

    std::cout << (elem.size() - 1) << '\n';
    for (int i = 1; i < elem.size(); i++) {
        std::cout << elem[i-1] << ' ' << elem[i] << '\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