Binary Tree Level Order Traversal
Difficulty: Medium
Category: DSA
Topics: Tree, Breadth-First Search, Binary Tree
Asked at: Bloomberg, Amazon, Facebook, Microsoft, LinkedIn, Adobe, Paypal, Google, Uber, Yahoo
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).
**Example 1:**
**Input:** root = [3,9,20,null,null,15,7]
**Output:** [[3],[9,20],[15,7]]
```
**Example 2:**
**Input:** root = [1]
**Output:** [[1]]
```
**Example 3:**
**Input:** root = []
**Output:** []
```
**Constraints:**
- The number of nodes in the tree is in the range `[0, 2000]`.
- `-1000 <= Node.val <= 1000`