Given the root of a binary tree, return the bottom-up level order traversal of its nodes' values (i.e., from left to right, level by level from leaf to root).
Example 1:
Input: root = [3, 9, 20, null, null, 15, 7]
Output: [[15, 7], [9, 20], [3]]
Explanation: The bottom-up level order traversal gives the deepest level first: [15, 7], then [9, 20], then [3].
Example 2:
Input: root = [1]
Output: [[1]]
Example 3:
Input: root = []
Output: []
[0, 2000]-1000 <= Node.val <= 1000