Binary Tree Level Order traversal in java

Table of Contents

Level Order traversal
Java Binary tree tutorial

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This is 5th part of java binary tree tutorial.

In this post, we will see about Level Order binary tree traversal in java.

Level Order traversal

Level order traversal of below binary tree will be:

We will use Queue for Level Order traversal.This algorithm is very similar to Breadth first search of graph.
Steps for Level order traversal algorithm:

Create empty queue and pust root node to it.
Do the following when queue is not empty
- Pop a node from queue and print it
- Push left child of popped node to queue if not null
- Push right child of popped node to queue if not null

// prints in level order

public static void levelOrderTraversal(TreeNode startNode) {

Queue<TreeNode> queue=new LinkedList<TreeNode>();

queue.add(startNode);

while(!queue.isEmpty())

{

TreeNode tempNode=queue.poll();

System.out.printf("%d ",tempNode.data);

if(tempNode.left!=null)

queue.add(tempNode.left);

if(tempNode.right!=null)

queue.add(tempNode.right);

}

Example:
Lets say your binary tree is :

So Level Order traversal will work as below:

Lets create java program for level order traversal:

package org.arpit.java2blog.binarytree;

import java.util.Queue;

import java.util.LinkedList;

public class BinaryTreeLevelOrder {

public static class TreeNode

{

int data;

TreeNode left;

TreeNode right;

TreeNode(int data)

{

this.data=data;

}

// prints in level order

public static void levelOrderTraversal(TreeNode startNode) {

Queue<TreeNode> queue=new LinkedList<TreeNode>();

queue.add(startNode);

while(!queue.isEmpty())

{

TreeNode tempNode=queue.poll();

System.out.printf("%d ",tempNode.data);

if(tempNode.left!=null)

queue.add(tempNode.left);

if(tempNode.right!=null)

queue.add(tempNode.right);

}

public static void main(String[] args)

{

// Creating a binary tree

TreeNode rootNode=createBinaryTree();

System.out.println("Level Order traversal of binary tree will be:");

levelOrderTraversal(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:

Level Order traversal of binary tree will be:
40 20 60 10 30 50 70

Java Binary tree tutorial

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Comments

Juan C. says:

March 16, 2016 at 8:45 pm

You have a typo after explain how Level Order traversal works on “Lets create java program for PreOrder traversal:” it is not PreOrder, anyway everything is well explained and is easy to see that is a typo.

Reply
Saket Kumar says:

April 20, 2017 at 6:12 am

Really good explanation

Reply
Kpd says:

August 3, 2017 at 10:13 pm

Good

Reply