Connect cities

#include <iostream>
#include <cmath>

#include <string>
#include <vector>
#include <set>
#include <utility>


void dfs(int s, std::vector<bool>& visited, std::vector<std::vector<int>>& graph) {
    if (visited[s]) return ;
    visited[s] = true;
    for (auto u : graph[s])
        dfs(u, visited, graph);
}

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

    if (n < 1 || n > 100000)
        return (0);
    if (m < 1 || m > 200000)
        return (0);

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

    std::vector<bool> visited(n + 1, false);
    std::vector<int> r={};

    for (int i = 1; i < n; ++i) {
        if (!visited[i]) {
            r.push_back(i);
            dfs(i, visited, graph);
        }
    }

    std::cout << r.size() - 1 << std::endl;
    for (size_t i = 0; i + 1 < r.size(); ++i) {
        std::cout << r[i] << " " << r[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: WRONG ANSWER

Test 12

Verdict: ACCEPTED