Level Order Traversal - Line Wise 4 : Using Pair class

I/P:

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 

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_Linewise_Of_GenericTree_4 {
    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;
    }

    private static class Pair {
        Node node;
        int level;

        Pair(Node node, int level) {
            this.node = node;
            this.level = level;
        }
    }

    public static void levelOrderLinewise(Node node) {
        Queue<Pair> mainQueue = new ArrayDeque<>();
        int level = 1;
        mainQueue.add(new Pair(node, level));

        while (!mainQueue.isEmpty()) {
            Pair pair = mainQueue.remove();
            if (pair.level > level) {
                level = pair.level;
                System.out.println();
            }
            System.out.print(pair.node.data + " ");

            for (Node child : pair.node.children) {
                Pair childPair = new Pair(child, level + 1);
                mainQueue.add(childPair);
            }
        }
    }


    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);
        levelOrderLinewise(root);
    }

}