Can someone please explain to me this adjacency list ?

Question

Hi, can someone please explain to me this adjacency list ? I understand the vectors and stuff but cant seem to comprehend adj[s].push_back(d) what does this mean ? Say s is 0 and d is 1 , will it create a new position and store the value 1 after 2 ? Or it will do this at the end, here is the code:

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

// Add edge
void addEdge(vector<int> adj[], int s, int d) {
adj[s].push_back(d);
adj[d].push_back(s);
}

int main() {
int V = 5;

// Create a graph
vector<int> adj[V];

// Add edges
addEdge(adj, 0, 1);
addEdge(adj, 0, 2);
addEdge(adj, 0, 3);
addEdge(adj, 1, 2);

}

Answer 1

adj[s].push_back(d) means:
create a directed edge from node s to d (s->d)

in that code, the edge is added both ways so the function is meant to create edges for undirected graph.

Answer 2

Thank you a lot !!