BFS in graph (code in JAVA)

import java.util.*;

public class graph {

    // Function to return Breadth First Traversal of given graph.

    public static ArrayList<Integer> bfsOfGraph(int numNodes, ArrayList<ArrayList<Integer>> adj) {

        ArrayList < Integer > bfs = new ArrayList < > (); // stores ans

        boolean vis[] = new boolean[numNodes + 1];

        Queue < Integer > q = new LinkedList < > ();

        // keep any node from where you want to start your bfs

        q.add(1); // keep value of node here, if your node has 0 then you should keep it

        vis[1] = true;

        while (!q.isEmpty()) {

            int levelSize = q.size(); // Number of nodes at the current level

            for (int i = 0; i < levelSize; i++) {

                Integer node = q.poll();

                bfs.add(node);

                for (Integer it : adj.get(node)) {

                    if (!vis[it]) {

                        vis[it] = true;

                        q.add(it);

                    }

                }

            }

            bfs.add(-1); // Add a marker (-1) to separate levels

        }

        return bfs;

    }

    public static void main(String[] args) {

        Scanner scanner = new Scanner(System.in);

        // Input the number of nodes.

        System.out.print("Enter the number of nodes: ");

        int numNodes = scanner.nextInt();

        // Create an adjacency list.

        ArrayList<ArrayList<Integer>> adjList = new ArrayList<>(numNodes);

        for (int i = 0; i < numNodes + 1; i++) { // numNodes + 2 only if remove empty zero index 

            adjList.add(new ArrayList<>());

        }

        // Input the number of edges.

        System.out.print("Enter the number of edges: ");

        int numEdges = scanner.nextInt();

        // Input the edges (connections between nodes).

        System.out.println("Enter the edges (startNode endNode):");

        for (int i = 0; i < numEdges; i++) {

            int startNode = scanner.nextInt();

            int endNode = scanner.nextInt();

            // Add an edge from startNode to endNode (for directed graph).

            adjList.get(startNode).add(endNode);

            // For undirected graph, add the reverse edge as well.

             adjList.get(endNode).add(startNode); // Uncomment this line for an undirected graph.

        }

        // Print the adjacency list.

        System.out.println("Adjacency List:");

        for (int i = 1; i < numNodes + 1; i++) {

            System.out.print(i + " -> ");

            for (int neighbor : adjList.get(i)) {

                System.out.print(neighbor + " ");

            }

            System.out.println();

        }

        // BFS of graph

        System.out.println("The bfs of the given graph is:");

        ArrayList<Integer> bfsResult = bfsOfGraph(numNodes, adjList);

        for (Integer node : bfsResult) {

            if (node == -1) {

                System.out.println(); // Print a newline for each level

            } else {

                System.out.print(node + " ");

            }

        }

        scanner.close();

    }

}