Level order traversal in spiral form

Printing tree in spiral form, or a zigzag form

would print 1 2 3 4 5 6 7

Algorithm

printSpiral(tree)
  set level = tree root
  while level contains nodes
     printLevel(level, ltr)
     set level = getNextLevel(level)
     ltr == !ltr 

Function to get next level

getNextLevel(level)
set nextLevel as empty list
foreach node in level
     if(node.left!=null) nextLevel.add(node.left)
     if(node.right!=null) nextLevel.add(node.right)

Function to print all nodes at a given level

printLevel(level, ltr)
if(!ltr)
    reverse level
foreach node in level
    print node.data

Example code in Java

package com.algorithms.treespiral;

import java.util.ArrayList;
import java.util.Collections;

public class Main {
    /* Driver program to test the above functions */
    public static void main(String[] args)
    {
        BinaryTree tree = new BinaryTree();
        tree.root = new Node(1);
        tree.root.left = new Node(2);
        tree.root.right = new Node(3);
        tree.root.left.left = new Node(7);
        tree.root.left.right = new Node(6);
        tree.root.right.left = new Node(5);
        tree.root.right.right = new Node(4);
        System.out.println("Spiral order traversal of Binary Tree is ");
        tree.printSpiral(tree.root);
    }
}

class Node
{
    int data;
    Node left, right;

    public Node(int d)
    {
        data = d;
        left = right = null;
    }
}

class BinaryTree
{
    Node root;

    /* print spiral order traversal of a tree  */
    void printSpiral(Node node)
    {
        boolean ltr = false;
        ArrayList<Node> layer = new ArrayList<>();
        layer.add(node);
        while(layer.size() != 0)
        {
            printLevel(layer,ltr);
            layer = getNextLevel(layer);
            ltr = !ltr;
        }
    }

    /* gets the next layer in the tree */
    ArrayList<Node> getNextLevel(ArrayList<Node> node) {
        ArrayList<Node> nextLayer = new ArrayList<>();
        for (Node n:node) {
            if(n.left != null)
                nextLayer.add(n.left);
            if(n.right != null) {
                nextLayer.add(n.right);
            }
        }
        return nextLayer;
    }

    /* prints tree level taking direction into account */
    void printLevel(ArrayList<Node> node, boolean ltr) {
        if (ltr != true) {
            Collections.reverse(node);
        }
        for (Node n:node) {
            System.out.print(n.data + " ");
        }
    }
}