Connect cities

#include <iostream>
#include <vector>
#include <unordered_map>
#include <unordered_set>

using namespace std;

// Function to perform DFS and find all nodes in a component
void dfs(int node, const unordered_map<int, vector<int>>& adj_list, vector<bool>& visited, vector<int>& component) {
    visited[node] = true;
    component.push_back(node);

    for (int neighbor : adj_list.at(node)) {
        if (!visited[neighbor]) {
            dfs(neighbor, adj_list, visited, component);
        }
    }
}

// Function to find all connected components
vector<vector<int>> find_connected_components(int n, const unordered_map<int, vector<int>>& adj_list) {
    vector<vector<int>> components;
    vector<bool> visited(n + 1, false);

    for (int i = 1; i <= n; ++i) {
        if (!visited[i]) {
            vector<int> component;
            dfs(i, adj_list, visited, component);
            components.push_back(component);
        }
    }

    return components;
}

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

    unordered_map<int, vector<int>> adj_list;
    for (int i = 0; i < m; ++i) {
        int a, b;
        cin >> a >> b;
        adj_list[a].push_back(b);
        adj_list[b].push_back(a);
    }

    // Find all connected components
    vector<vector<int>> components = find_connected_components(n, adj_list);

    if (components.size() == 1) {
        // The graph is already connected
        cout << 0 << endl;
        return 0;
    }

    // Output the minimum number of roads required
    cout << components.size() - 1 << endl;

    // Connect the components
    vector<int> component_representatives;
    for (const auto& component : components) {
        component_representatives.push_back(component[0]);
    }

    for (size_t i = 0; i < component_representatives.size() - 1; ++i) {
        cout << component_representatives[i] << " " << component_representatives[i + 1] << endl;
    }

    return 0;
}

Test 1

Verdict: RUNTIME ERROR

terminate called after throwing an instance of 'std::out_of_range'
  what():  _Map_base::at

Test 2

Verdict: RUNTIME ERROR

terminate called after throwing an instance of 'std::out_of_range'
  what():  _Map_base::at

Test 3

Verdict: ACCEPTED

Test 4

Verdict: RUNTIME ERROR

terminate called after throwing an instance of 'std::out_of_range'
  what():  _Map_base::at

Test 5

Verdict: ACCEPTED

Test 6

Verdict: RUNTIME ERROR

terminate called after throwing an instance of 'std::out_of_range'
  what():  _Map_base::at

Test 7

Verdict: RUNTIME ERROR

terminate called after throwing an instance of 'std::out_of_range'
  what():  _Map_base::at

Test 8

Verdict: RUNTIME ERROR

terminate called after throwing an instance of 'std::out_of_range'
  what():  _Map_base::at

Test 9

Verdict: RUNTIME ERROR

terminate called after throwing an instance of 'std::out_of_range'
  what():  _Map_base::at

Test 10

Verdict: RUNTIME ERROR

terminate called after throwing an instance of 'std::out_of_range'
  what():  _Map_base::at

Test 11

Verdict: RUNTIME ERROR

terminate called after throwing an instance of 'std::out_of_range'
  what():  _Map_base::at

Test 12

Verdict: RUNTIME ERROR

terminate called after throwing an instance of 'std::out_of_range'
  what():  _Map_base::at