Connect cities

#include <iostream>
#include <vector>

using namespace std;

void dfs(vector<vector<int>> &adj, vector<bool> &visited, int s, vector<int> &res) {
    visited[s] = true;

    res.push_back(s);

    int connections = adj[s].size();
    for (int i = 0; i < connections; ++i) {
        int c = adj[s][i];
        if (visited[c] == false) dfs(adj, visited, c, res);
    }
}

int first_not_connected(vector<bool> &visited) {
    int i = 1;
    while(visited[i] == true) ++i;
    return i;
}

void algorithm(vector<vector<int>> &adj, vector<bool> &visited, int s, vector<int> &res, int &count, vector<pair<int,int>> &new_roads) {
    dfs(adj, visited, s, res);
    while(res.size() != (visited.size() - 1)) {
        int last_visited = res[res.size()-1];
        s = first_not_connected(visited);
        new_roads.push_back(make_pair(s, last_visited));
        count++;
        adj[s].push_back(last_visited);
        adj[last_visited].push_back(s);
        dfs(adj, visited, s, res);
    }
}

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

    vector<vector<int>> adj(n+1);

    // Read the existing roads
    for (int i = 0; i < m; ++i) {
        int c1, c2;
        cin >> c1 >> c2;
        adj[c1].push_back(c2);
        adj[c2].push_back(c1);
    }

    // DFS
    vector<bool> visited(n+1, false);
    vector<int> res;
    int count = 0;
    vector<pair<int,int>> new_roads;
    algorithm(adj, visited, 1, res, count, new_roads);

    cout << count << endl;
    for(int i = 0; i < count; ++i) cout << new_roads[i].first << " " << new_roads[i].second << 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: TIME LIMIT EXCEEDED

Test 12

Verdict: ACCEPTED