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).
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? > read more on how binary tree is serialized on OJ.

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