mediumTrees

Binary Tree Level Order Traversal

## Problem

Given the root of a binary tree, return the **level order traversal** of its nodes' values (i.e., from left to right, level by level).

> **Input format:** Level-order list where `None` represents a missing node.

Examples

Input
root = [3,9,20,null,null,15,7]
Output
[[3],[9,20],[15,7]]
Level 0: [3], Level 1: [9,20], Level 2: [15,7].
Input
root = [1]
Output
[[1]]
Single node.
Input
root = []
Output
[]
Empty tree.

Constraints

The number of nodes in the tree is in the range [0, 2000]. -1000 <= Node.val <= 1000
Python
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