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]
]
Tags: Tree, Breadth-first Search
题意是从下往上按层遍历二叉树，每一层是从左到右，按层遍历
宽搜就行了，因为是从下往上，所以插入的时候每次插到链表头即可。
思路2 ```go
```
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