Problem

Given the root node of a binary tree, return an array of array, where each subarray contains the values of the tree's nodes at each level from top to bottom and left to right.

Example 1

treeByLevels([1,2,3,null,4,5])
// return [[1], [2,3], [4,5]]

Example 2

treeByLevels([1,2,3,null,null,null,4,5])
// return [[1], [2,3], [4], [5]]