Question 57 : Write an algorithm to do level order traversal of binary tree?

You need to write java program to do level order traversal of binary tree. You can use queue data structure to do level order traversal.

Level order traversal of binary tree  will be below:

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:

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 the above program and you will get the following output:

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

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