Reverse level order traversal of binary tree in java

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This is part of java binary tree tutorial.
In this post, we will see about Reverse Level Order binary tree traversal in java. In previous post, we have already seen Level order traversal. In reverse level order traversal, we will visit last level first, then second last and eventually first level.

Reverse Level Order traversal:

Reverse Level order traversal of below tree will be:


We will use stack  for Reverse Level Order traversal.

Steps for Reverse Level order traversal algorithm:
  1. Create empty queue and push root node to it.
  2. Do the following when queue is not empty
    • Pop a node from queue and print it
    • Push right child of popped node to queue if not null
    • Push left child of popped node to queue if not null
    • Push current node to stack
    • Pop node from stack and print it
 
// prints in reverse level order
 public static void reverseLevelOrderTraversal(TreeNode startNode) {
  Queue<TreeNode> queue=new LinkedList<TreeNode>();
  Stack<TreeNode> stack=new Stack<TreeNode>();
  queue.add(startNode);
  while(!queue.isEmpty())
  {
   TreeNode tempNode=queue.poll();
   if(tempNode.right!=null)
    queue.add(tempNode.right);
   if(tempNode.left!=null)
    queue.add(tempNode.left);
   
   stack.push(tempNode);
  }
  
  while(!stack.empty())
   System.out.print(stack.pop().data+" " );
 }

Example:
Lets say your binary tree is :

So Reverse Level Order traversal will work as below:

Lets create java program :
 

package org.arpit.java2blog;

import java.util.LinkedList;
import java.util.Queue;
import java.util.Stack;
public class BinaryTree {


 public static class TreeNode
 {
  int data;
  TreeNode left;
  TreeNode right;
  TreeNode(int data)
  {
   this.data=data;
  }
 }

 // prints in reverse level order
 public static void reverseLevelOrderTraversal(TreeNode startNode) {
  Queue<TreeNode> queue=new LinkedList<TreeNode>();
  Stack<TreeNode> stack=new Stack<TreeNode>();
  queue.add(startNode);
  while(!queue.isEmpty())
  {
   TreeNode tempNode=queue.poll();
   if(tempNode.right!=null)
    queue.add(tempNode.right);
   if(tempNode.left!=null)
    queue.add(tempNode.left);
   
   stack.push(tempNode);
  }
  
  while(!stack.empty())
   System.out.print(stack.pop().data+" " );
 }
 public static void main(String[] args)
 {
  BinaryTree bi=new BinaryTree();
  // Creating a binary tree
  TreeNode rootNode=createBinaryTree();
  System.out.println("Reverse Level Order traversal of binary tree will be:");
  reverseLevelOrderTraversal(rootNode);
 }

 public static TreeNode createBinaryTree()
 {

  TreeNode rootNode =new TreeNode(40);
  TreeNode node20=new TreeNode(20);
  TreeNode node10=new TreeNode(10);
  TreeNode node30=new TreeNode(30);
  TreeNode node60=new TreeNode(60);
  TreeNode node50=new TreeNode(50);
  TreeNode node70=new TreeNode(70);

  rootNode.left=node20;
  rootNode.right=node60;

  node20.left=node10;
  node20.right=node30;

  node60.left=node50;
  node60.right=node70;

  return rootNode;
 }
}


Run above program and you will get following output:
 
Reverse Level Order traversal of binary tree will be:
10 30 50 70 20 60 40

Java Binary tree tutorial:

    Binary tree in java Binary tree preorder traversal Binary tree postorder traversal Binary tree inorder traversal Binary tree level order traversal Binary tree spiral order traversal Binary tree reverse level order traversal Binary tree boundary traversal Print leaf nodes of binary tree Count leaf nodes in binary tree get maximum element in binary tree Print all paths from root to leaf in binary tree Print vertical sum of binary tree in java Get level of node in binary tree in java Lowest common ancestor(LCA) in binary tree in java

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Written by Arpit:

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