Problem statement 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). Problem statement taken from: https://leetcode.com/problems/binary-tree-level-order-traversal-ii/ Example 1: Input: root = [3, 9, 20, null, null, 15, 7]
Output: [[15, 7], [9, 20]…