Connect cities

input/code.cpp: In function 'void dfs(const std::vector<std::vector<int> >&, std::vector<bool>&, std::vector<int>&)':
input/code.cpp:18:23: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<std::vector<int> >::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   18 |     for (int i = 0; i < adj.size(); i++)
      |                     ~~^~~~~~~~~~~~
input/code.cpp: In function 'int main()':
input/code.cpp:54:27: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   54 |         for (int i = 1; i < connectedComps.size(); i++)
      |                         ~~^~~~~~~~~~~~~~~~~~~~~~~

#include <iostream>
#include <vector>
using namespace std;


void dfs_rec(int s, const vector<vector<int>>& adj, vector<bool>& vis)
{
    if (vis[s]) return;
    vis[s] = true;
    for (auto u : adj[s])
    {
        dfs_rec(u, adj, vis);
    }
}

void dfs(const vector<vector<int>>& adj, vector<bool>& vis, vector<int>& connectedComps)
{
    for (int i = 0; i < adj.size(); i++)
    {
        if (vis[i]) continue;
        else
        {
            connectedComps.push_back(i);
            dfs_rec(i, adj, vis);
        }
    }
    return;
}

int main()
{
    int numCities, numRoads;
    cin >> numCities >> numRoads;
    vector<vector<int>> adj(numCities);
    vector<bool> vis(numCities, false);
    vector<int> connectedComps;
    for (int i = 0; i < numRoads; i++)
    {
        int from, to;
        cin >> from >> to;
        from--;
        to--;
        adj[from].push_back(to);
        adj[to].push_back(from);
    }
    dfs(adj, vis, connectedComps);
    if (connectedComps.size() == 0)
    {
        cout << "ERROR" << endl;
    }
    else
    {
        cout << connectedComps.size() - 1 << endl;
        for (int i = 1; i < connectedComps.size(); i++)
        {
            cout << connectedComps[0] + 1 << " " << connectedComps[i] + 1 << 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