问题描述：
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]
]注意是每一层的所有数字放入同一个list内
思路：二叉树问题，考虑使用递归算法，计算出每一层的所有元素值
# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = Noneclass Solution:
def levelOrderBottom(self, root: TreeNode) -> List[List[int]]:
if root == None: return []
def order(rootx,level):
if rootx == None:return
if (level) == len(result):
result.append([rootx.val])
else:
result[level].append(rootx.val)
order(rootx.left,level+1)
order(rootx.right,level+1) result = []
order(root,0)
return result[::-1]
