Binary Tree Level Order Traversal

Given the root of a binary tree, return its values level by level: a list of rows, top to bottom, each row listed left to right.

Example 1

Input: root = [3, 9, 20, None, None, 15, 7]
Output: [[3], [9, 20], [15, 7]]

Example 2

Input: root = [1]
Output: [[1]]

Example 3

Input: root = []
Output: []

Constraints

• 0 ≤ number of nodes ≤ 2 · 10^4
• -10^5 ≤ Node.val ≤ 10^5