Reverse Level Order Tree Traversal

Given the root node of a binary tree, return the bottom-up level order traversal of the node values. This means you should traverse the tree level by level, but present the levels in reverse order—starting from the deepest (leaf) level and working upward to the root.

Within each level, nodes should be listed from left to right as they appear in the tree. If the tree is empty (i.e., the root is null), return an empty list.

Understanding the Traversal:

• Standard level order traversal (BFS) visits nodes level-by-level from the root downward.
• This problem requires the same grouping by levels, but the final result should present the deepest level first and the root level last.
• Each inner list in the result represents all node values at a particular depth in the tree.

Tree Representation: The binary tree is provided in level-order format as an array where:

• The first element is the root
• For a node at index i, its left child is at index 2i+1 and right child is at index 2i+2
• null values represent absent nodes