BottomUp LevelOrder Binary Tree

Given a binary tree, return the bottom-up level order traversal of its
nodes' values. (ie, from left to right, level by level from leaf to
root).

For example:
Given binary tree {3,9,20,#,#,15,7},
3
/ \
9  20
/  \
15   7
return its bottom-up level order traversal as:
[
[15,7],
[9,20],
[3]
]