spiral BT java

// http://code2begin.blogspot.com
// program to print given binary tree in spiral level wise traversal

/**
 * Created by MOHIT on 25-05-2018.
 */

import java.util.*;

// node class
class node{
    int data;
    node left;
    node right;

    // function that returns a pointer to new node
    public node(int element){
        this.data = element;
        this.left = null;
        this.right = null;

    }
};

public class BinaryTree {

    // function to print the tree using spiral level wise traversal
    static void spiral_traversal(node root){
        if (root == null){
            return;
        }

        ArrayList STACK1 = new ArrayList<node>();
        ArrayList STACK2 = new ArrayList<node>();

        int level = 1;
        STACK1.add(root);
        while(!STACK1.isEmpty() || !STACK2.isEmpty()){
            System.out.print("\nNodes at level " + level + " are : ");
            level += 1;
            while (!STACK1.isEmpty()){
                // pop the first node from the queue
                node NODE = (node)STACK1.get(0);
                STACK1.remove(0);
                System.out.print(NODE.data + " ");

                // push the left child on queue
                if (NODE.right != null) {
                    STACK2.add(NODE.right);
                }

                // push the right child on queue
                if (NODE.left != null) {
                    STACK2.add(NODE.left);
                }
            }

            System.out.print("\nNodes at level " + level + " are : ");
            level += 1;
            while (!STACK2.isEmpty()){
                // pop the first node from the queue
                node NODE = (node)STACK2.get(0);
                STACK2.remove(0);
                System.out.print(NODE.data + " ");

                // push the left child on queue
                if (NODE.right != null) {
                    STACK1.add(NODE.right);
                }

                // push the right child on queue
                if (NODE.left != null) {
                    STACK1.add(NODE.left);
                }
            }
        }
    }

    public static void main(String arg[]) {
        node head = new node(1);
        head.left = new node(2);
        head.right = new node(3);
        head.left.left = new node(4);
        head.left.right = new node(5);
        head.right.right = new node(6);
        head.left.left.right = new node(7);
        head.right.right.left = new node(8);
        head.left.left.right.left = new node(9);
        head.left.left.right.left.left = new node(10);
        head.right.right.left.right = new node(11);

        System.out.print("SPIRAL Level wise traversal of the above binary tree is : ");
        spiral_traversal(head);
    }
}