从底向上层次遍历二叉树

题目原型：

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}

,

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return its bottom-up level order traversal as:

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基本思路：

由于是从底向上，所以用到了栈。

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