Connect cities

// ~/.vim/cpp_template.cpp
#include <bits/stdc++.h>
#include <iostream>
#include <vector>
#include <algorithm>


using namespace std;


void dfs(int s, vector<bool> *visited, vector<int> (*adj)[]) {
    if ((*visited)[s]) return;
    (*visited)[s] = true;
    // process node s


    for (auto u: (*adj)[s]) {
        dfs(u, visited, adj);
    }
}
//vector<int> adj[N];                                                          
//vector<bool> visited(N, false); 

int main() {
    ios::sync_with_stdio(0);
    cin.tie(0);
    // Your code starts here

    int N, M;
    cin >> N >> M;



    vector<int> adj[N];
    vector<bool> visited(N, false);


    int u, v;
    for(int i = 0; i < M;i++){
        cin >> u >> v;
        u--;
        v--;
        adj[u].push_back(v);
        adj[v].push_back(u);
    }

    int res = 0;
    vector<int> build_roads;
    dfs(0, &visited, &adj);
    for(int i = 0; i < N; i++){
        if(visited[i]) continue;
        dfs(i, &visited, &adj);
        //cout << 1 << ' ' << i + 1 << endl;
        build_roads.push_back(i+1);
        res++;
    }
    cout << res << endl;
    for(auto i = 0u; i < build_roads.size();i++){
        cout << 1 << ' ' << build_roads[i] << 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