Binary Tree Level Order traversal in java

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This is 5th part of java binary tree tutorial.

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 Print leaf nodes of binary tree Count leaf nodes 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
In this post, we will see about Level Order binary tree traversal in java.

Level Order traversal:

Level order traversal of below 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:
  1. Create empty queue and pust root node to it.
  2. 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;

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

 
 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)
 {
  BinaryTree bi=new BinaryTree();
  // 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 

Please go through java interview programs for more such programs.

Written by Arpit:

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