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.
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.
Example
Input: [3,9,20,null,null,15,7]
Output: [[3],[9,20],[15,7]]