1. Introduction

Level order traversal, also known as Breadth-first search (BFS) for trees, is a technique where you traverse the tree level by level. Starting from the root, you move through each level from left to right before advancing to the subsequent level. This traversal approach is particularly advantageous in scenarios where actions must be taken by tree levels or when a solution closer to the root is preferable.

2. Program Steps

1. Create a Queue to facilitate the BFS traversal.

2. Insert the root of the binary tree into the Queue.

3. While the Queue isn’t empty, remove the front node and print its value.

4. Insert the left child of the current node into the Queue if it exists.

5. Insert the right child of the current node into the Queue if it exists.

6. Repeat steps 3-5 until the Queue becomes empty.

3. Code Program

import java.util.LinkedList;
import java.util.Queue;

class TreeNode {
    int value;
    TreeNode left;
    TreeNode right;

    TreeNode(int value) {
        this.value = value;
        left = null;
        right = null;
    }
}

public class LevelOrderTraversal {

    public static void levelOrder(TreeNode root) {
        if (root == null) return;

        // Create an empty queue for level order traversal
        Queue<TreeNode> queue = new LinkedList<>();

        // Enqueue root and initialize traversal
        queue.add(root);

        while (!queue.isEmpty()) {
            // Print front of queue and remove it from queue
            TreeNode tempNode = queue.poll();
            System.out.print(tempNode.value + " ");

            // Enqueue left child
            if (tempNode.left != null) {
                queue.add(tempNode.left);
            }

            // Enqueue right child
            if (tempNode.right != null) {
                queue.add(tempNode.right);
            }
        }
    }

    public static void main(String[] args) {
        TreeNode root = new TreeNode(1);
        root.left = new TreeNode(2);
        root.right = new TreeNode(3);
        root.left.left = new TreeNode(4);
        root.left.right = new TreeNode(5);

        System.out.println("Level order traversal of binary tree is: ");
        levelOrder(root);
    }
}

Output:

Level order traversal of binary tree is:
1 2 3 4 5

4. Step By Step Explanation

1. The TreeNode class represents individual nodes in the binary tree, each having a value, a left child, and a right child.

2. We use a Queue (specifically LinkedList in this case) to help facilitate the BFS traversal.

3. levelOrder function represents the main logic for traversal. The root is first added to the queue.

4. Inside the while loop, as long as the queue isn't empty, we keep removing (or polling) the front node of the queue and printing its value.

5. After printing, we add the left and right children of the current node to the queue if they exist. This ensures that nodes are printed level by level, from left to right.

6. The main method demonstrates a simple binary tree and its level order traversal.