Connect cities

#include <iostream>
#include <vector>

using namespace std;

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

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

void print_results(vector<int> &cc) {
    int roads = cc.size() - 1;
    cout << roads << endl;

    for (int i = 0; i < roads; ++i) {
        cout << cc[i] << " " << cc[i+1] << endl;
    }
}

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);
    }

    // With more than (n-1)*(n-2) / 2 edges the graph is connected
    if (m > (((n-1)*(n-2))/2)) {
        cout << 0 << endl;
    }
    else {
        vector<bool> visited(n+1, false);
        vector<int> cc;
        
        for (int i = 1; i <= n; ++i) {
            if (!visited[i]) {
                cc.push_back(i);
                dfs(adj, visited, i);
            }
        }
        
        print_results(cc);
    }

}

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