Connect cities

#include "iostream"
#include "vector"
#include "map"
#include "list"
#include "algorithm"


int main() {
    int n, m;
    std::cin >> n >> m;
    if (1 > n || n > 100000 || 1 > m || m > 200000) return EXIT_FAILURE;
    std::map<int, std::vector<int>> connections;
    int a, b;
    for (int i = 0; i < m; i ++) {
        std::cin >> a >> b;
        if ((1 <= a) && (a <= n) && (1 <= b) && (b <= n)) {
            connections[a].push_back(b);
            connections[b].push_back(a);
        }
    }
    std::vector<bool> visited(n + 1, false);
    std::vector<int> heads;
    for (int i = 1; i <= n; i++) {
        if (!visited[i]) {
            heads.push_back(i);
            std::list<int> neighbors;
            neighbors.push_back(i);
            visited[i] = true;
            while (!neighbors.empty()) {
                int node = neighbors.front();
                neighbors.pop_front();
                for (int neighbor : connections[node]) {
                    if (!visited[neighbor]) {
                        visited[neighbor] = true;
                        neighbors.push_back(neighbor);
                    }
                }
            }
        }
    }
    int k = heads.size() - 1;
    if (k < 0) k = 0;
    std::cout << k << std::endl;
    for (int i = 0;  i < k; i++) {
        std::cout << heads[i] << " " << heads[i+1] << std::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: ACCEPTED

Test 12

Verdict: ACCEPTED