Question:

 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] 
  ]