Problem

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,null,null,15,7],

3

/
9 20 /
15 7

return its bottom-up level order traversal as:

[ [15,7], [9,20], [3] ]

Pre analysis

Will simply traverse recursively.