Binary Tree Level Order Traversal II

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]
]
confused what "{1,#,2,3}" means?