Traversals in Binary Tree

package pep.Day31;

import java.util.Stack;

public class Traversals_in_BinaryTree {
    private static class Pair {
        Node node;
        int state;

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

    private static class Node {
        int data;
        Node left;
        Node right;

        Node(int data, Node left, Node right) {
            this.data = data;
            this.left = left;
            this.right = right;
        }

    }

    public static Node construct(Integer[] arr) {
        Stack<Pair> stack = new Stack<>();
        Node root = new Node(arr[0], null, null);
        Pair rootPair = new Pair(root, 1);
        stack.push(rootPair);

        int idx = 0;
        while (stack.size() > 0) {
            Pair top = stack.peek();
            if (top.state == 1) {
                idx++;
                if (arr[idx] != null) {
                    top.node.left = new Node(arr[idx], null, null);
                    Pair lp = new Pair(top.node.left, 1);
                    stack.push(lp);
                } else {
                    top.node.left = null;
                }

                top.state++;
            } else if (top.state == 2) {
                idx++;
                if (arr[idx] != null) {
                    top.node.right = new Node(arr[idx], null, null);
                    Pair rp = new Pair(top.node.right, 1);
                    stack.push(rp);
                } else {
                    top.node.right = null;
                }

                top.state++;
            } else {
                stack.pop();
            }
        }
        return root;
    }

    public static void display(Node node) {
        if (node == null)
            return;
        String str = "";
        str += node.left == null ? "." : node.left.data;
        str += " <- " + node.data + " -> ";
        str += node.right == null ? "." : node.right.data;

        System.out.println(str);
        display(node.left);
        display(node.right);
    }

    public static void main(String[] args) {
        Integer[] arr = {50, 25, 12, null, null, 37, 30, null, null, null,
                75, 62, null, 70, null, null, 87, null, null};

        Node root = construct(arr);
        //display(root);

        traversal(root);

    }

    private static void traversal(Node node) {
        if (node == null)
            return;

        System.out.println(node.data + " in pre-order");  //euler left - pre
        traversal(node.left);
        System.out.println(node.data + " in in-order");  //euler between - in
        traversal(node.right);
        System.out.println(node.data + " in post-order");  //euler right - post
    }
}