vertical order bt java

// http://code2begin.blogspot.com
// program to print the given binary tree in vertical order

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

import java.io.*;
import java.lang.reflect.Array;
import java.util.*;

import static java.lang.Integer.max;


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

    // function that returns a pointer to new node
    public node(int element){
        this.data = element;
        this.hd = -1;
        this.left = null;
        this.right = null;

    }
};


public class BinaryTree {

    // function to print the nodes of the binary tree in vertical order
    // using horizontal distance of a node from the root node
    static void vertical_order(node root){
        if(root == null){
            return;
        }

        // initialize a map and queue
        // we will use iterative BFS traversal to solve this
        Map M = new HashMap<Integer, Integer>();
        ArrayList<ArrayList<Integer>> A = new ArrayList();
        ArrayList Q = new ArrayList<node>();;

        // push root in queue and initialize horizontal distance
        Q.add(root);
        int horizontal_distance = root.hd;

        while(!Q.isEmpty()){
            node current = (node)Q.get(0);
            Q.remove(0);
            if(M.containsKey(current.hd)){
                int index = (int)M.get(current.hd);
                A.get(index).add(current.data);
            }else{
                A.add(new ArrayList<>());
                int index = A.size() - 1;
                A.get(index).add(current.data);
                M.put(current.hd, index);
            }

            // update horizontal distance of the left child and put it in the queue
            if(current.left != null){
                current.left.hd = current.hd - 1;
                Q.add(current.left);
            }
            // update horizontal distance of the right child and put it in the queue
            if(current.right != null) {
                current.right.hd = current.hd + 1;
                Q.add(current.right);
            }
        }

        List<Integer> a = new ArrayList<>(M.keySet());
        for(int hd:a){
            int index = (int)M.get(hd);
            System.out.println("Nodes at horizontal distance " + hd + " are : " + A.get(index));
        }
    }

    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);
        head.hd = 0;

        System.out.println("Vertical order traversal of the given tree is : ");
        vertical_order(head);

    }
}