1. Introduction

"Binary Tree Level Order Traversal" is a classic problem in tree data structures that involves traversing a binary tree level by level from left to right. This problem is essential for understanding breadth-first traversal in trees and is a common challenge in coding interviews.

Problem

Given the root of a binary tree, the task is to return the level order traversal of its nodes' values, i.e., traversing the nodes from left to right at each level.

2. Solution Steps

1. Utilize a queue to keep track of nodes at each level of the tree.

2. Start with the root node and then process each level of the tree one by one.

3. For each node at a given level, add its left and right children to the queue.

4. Continue this process until all levels of the tree have been traversed.

5. Keep track of the nodes' values at each level and return the traversal result.

3. Code Program

class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right

from collections import deque

def levelOrder(root):
    if not root:
        return []

    result = []
    queue = deque([root])

    while queue:
        level_size = len(queue)
        level = []

        for _ in range(level_size):
            node = queue.popleft()
            level.append(node.val)

            if node.left:
                queue.append(node.left)
            if node.right:
                queue.append(node.right)

        result.append(level)

    return result

# Example Usage
root = TreeNode(3)
root.left = TreeNode(9)
root.right = TreeNode(20, TreeNode(15), TreeNode(7))
print(levelOrder(root))