Level Order traversal of a generic tree(n-ary tree) in java

Question

(In case you want to avoid the lengthy explanation, all I am looking for is a level order traversal for a generic-tree(n-ary tree) in java. The code supplied works and needs

Answer 1

Level-order traversal using Queue:

import java.util.ArrayList;
import java.util.Arrays;
import java.util.LinkedList;
import java.util.List;
import java.util.Objects;
import java.util.Queue;
import java.util.stream.Collectors;

public class LevelOrderTraversal {
    static class Node {
        int data;

        Node children[];

        Node(int data, int n) {
            children = new Node[n];
            this.data = data;
        }
    }

    public static void main(String[] args) {
        /*  
                       1 
                    /  |  \ 
                   2   3   4 
                 / | \ 
                5  6  7 
        */
        int n = 3;
        Node root = new Node(1, n);
        root.children[0] = new Node(2, n);
        root.children[1] = new Node(3, n);
        root.children[2] = new Node(4, n);
        root.children[0].children[0] = new Node(5, n);
        root.children[0].children[1] = new Node(6, n);
        root.children[0].children[2] = new Node(7, n);

        List> levelList = levelOrder(root);
        for (List level : levelList) {
            for (Integer val : level) {
                System.out.print(val + " ");
            }
            System.out.println();
        }
    }

    public static List> levelOrder(Node root) {
        List> levelList = new ArrayList<>();

        if (root == null) {
            return levelList;
        }

        Queue queue = new LinkedList<>();
        queue.add(root);

        while (!queue.isEmpty()) {
            int n = queue.size();
            List level = new ArrayList<>();

            while (n-- > 0) {
                Node node = queue.remove();
                level.add(node.data);
                queue.addAll(Arrays.stream(node.children).filter(Objects::nonNull).collect(Collectors.toList()));
            }
            levelList.add(level);
        }
        return levelList;
    }

}