Level-order Of Generic Tree / BREADTH FIRST TRAVERSAL

I/O: 

24 10 20 50 -1 60 -1 -1 30 70 -1 80 110 -1 120 -1 -1 90 -1 -1 40 100 -1 -1 -1

O/P:

10 20 30 40 50 60 70 80 90 100 110 120 .

RPA: Remove, Print and Add to operate the queue. This rule says that, remove the element from the queue, print that element and then add its children to the queue.

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.ArrayDeque;
import java.util.ArrayList;
import java.util.Queue;
import java.util.Stack;

public class LevelOrder_Of_GenericTree {
    private static class Node {
        int data;
        ArrayList<Node> children = new ArrayList<>();
    }

    public static void display(Node node) {
        String str = node.data + " -> ";
        for (Node child : node.children) {
            str += child.data + ", ";
        }
        str += ".";
        System.out.println(str);

        for (Node child : node.children) {
            display(child);
        }
    }

    public static Node construct(int[] arr) {
        Node root = null;

        Stack<Node> st = new Stack<>();
        for (int i = 0; i < arr.length; i++) {
            if (arr[i] == -1) {
                st.pop();
            } else {
                Node t = new Node();
                t.data = arr[i];

                if (st.size() > 0) {
                    st.peek().children.add(t);
                } else {
                    root = t;
                }

                st.push(t);
            }
        }

        return root;
    }

    public static int size(Node node) {
        int s = 0;

        for (Node child : node.children) {
            s += size(child);
        }
        s += 1;

        return s;
    }

    public static void levelOrder(Node node) {
        // add parent node
        Queue<Node> queue = new ArrayDeque<>();
        queue.add(node);
        while (!queue.isEmpty()) {
            // remove
            Node out = queue.remove();

            // print
            System.out.print(out.data + " ");

            // add child
            for (Node child : out.children) {
                queue.add(child);
            }

        }
        System.out.println(".");
    }

    public static void main(String[] args) throws Exception {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        int n = Integer.parseInt(br.readLine());
        int[] arr = new int[n];
        String[] values = br.readLine().split(" ");
        for (int i = 0; i < n; i++) {
            arr[i] = Integer.parseInt(values[i]);
        }
        Node root = construct(arr);
        levelOrder(root);
    }

}

Time Complexity:

O(n) We travel to all the elements of the tree. Every node is processed individually in a while loop.

Space Complexity:

O(n) A queue of size n will be used.