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), as an array of arrays — one per level.
Trees in this problem are given as a level-order array: null marks a missing child, and trailing nulls are omitted.
Example:
Input: root = [3,9,20,null,null,15,7]
Output: [[3],[9,20],[15,7]]