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 / \ 920 / \ 157

return its bottom-up level order traversal as:

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

按层序从下到上遍历一颗树。

用一个队列即可实现。