Light rail connections

input/code.cpp: In function 'void dfs(long long int, long long int, std::vector<std::vector<long long int> >&, std::vector<long long int>&, std::vector<std::pair<long long int, long long int> >&, long long int)':
input/code.cpp:21:23: warning: comparison of integer expressions of different signedness: 'long long int' and 'std::vector<long long int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   21 |     for (int i = 0; i < adj[start].size(); i++) {
      |                     ~~^~~~~~~~~~~~~~~~~~~
input/code.cpp: In function 'void solve()':
input/code.cpp:54:23: warning: comparison of integer expressions of different signedness: 'long long int' and 'std::vector<long long int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   54 |     for (int v = 1; v < visited.size(); v++)
      |                     ~~^~~~~~~~~~~~~~~~
input/code.cpp:61:23: warning: comparison of integer expressions of different signedness: 'long long int' and 'std::vector<long long int>::size_type'...

#include <bits/stdc++.h>
#include <iostream>

#define int long long
using namespace std;

void dfs(int start, int prev, vector<vector<int>> &adj, vector<int> &visited, vector<pair<int, int>> &edges, int prev_ind)
{
    if(visited[start] == 1) {
        return;
    }
    visited[start] = 1;

    // remove edge from structure
    if (prev_ind != -1) {
        adj[prev][prev_ind] = -1;
        edges.push_back({prev, start});
    }
    // add edge to saved edges

    for (int i = 0; i < adj[start].size(); i++) {
        if (adj[start][i] == prev) {
            // remove duplicate edges
            adj[start][i] = -1;
            continue;
        }
        if (adj[start][i] != -1)
            dfs(adj[start][i], start, adj, visited, edges, i);
    }
}

void solve()
{
    int n, m;
    cin >> n >> m;

    vector<int> visited(n + 1, 0);
    vector<vector<int>> adj(n + 1, vector<int>());
    int a, b;
    for (int i = 0; i < m; i++) {
        cin >> a >> b;
        adj[a].push_back(b);
        adj[b].push_back(a);
    }

    /*for (auto u : adj) {
        for (auto el : u) {
            cout << el << " ";
        }
        cout << "\n";
    }*/
    vector<pair<int, int>> edges;
    // dfs(1, 1, adj, visited, edges, 0);
    for (int v = 1; v < visited.size(); v++) 
    {
        if (visited[v] == 0) {
            dfs(v, v, adj, visited, edges, -1);
        }
    }
    visited = vector<int>(n + 1, 0);
    for (int v = 1; v < visited.size(); v++) 
    {
        if (visited[v] == 0) {
            dfs(v, v, adj, visited, edges, -1);
        }
    }
    visited = vector<int>(n + 1, 0);
    for (int v = 1; v < visited.size(); v++) 
    {
        if (visited[v] == 0) {
            dfs(v, v, adj, visited, edges, -1);
        }
    }
    cout << edges.size() << "\n";
    for (auto u : edges) {
        cout << u.first << " " << u.second << "\n";
    }
}

signed main()
{
    int t = 1;
    // cin >> t;
    for (int i = 0; i < t; i++)
        solve();
}

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

Test 13

Verdict: ACCEPTED

Test 14

Verdict: ACCEPTED

Test 15

Verdict: ACCEPTED

Test 16

Verdict: ACCEPTED

Test 17

Verdict: ACCEPTED

Test 18

Verdict: ACCEPTED

Test 19

Verdict: ACCEPTED

Test 20

Verdict: ACCEPTED

Test 21

Verdict: ACCEPTED

Test 22

Verdict: ACCEPTED

Test 23

Verdict: ACCEPTED

Test 24

Verdict: ACCEPTED

Test 25

Verdict: ACCEPTED

Test 26

Verdict: ACCEPTED

Test 27

Verdict: ACCEPTED

Test 28

Verdict: ACCEPTED

Test 29

Verdict: ACCEPTED

Test 30

Verdict: ACCEPTED

Test 31

Verdict: ACCEPTED

Test 32

Verdict: ACCEPTED

Test 33

Verdict: ACCEPTED

Test 34

Verdict: ACCEPTED

Test 35

Verdict: ACCEPTED

Test 36

Verdict: ACCEPTED

Test 37

Verdict: ACCEPTED

Test 38

Verdict: ACCEPTED