题目
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).
解题思路
从根节点开始dfs，把每一层放到结果里，同时求出下一层的所有节点
代码
class Solution:
def levelOrderBottom(self, root: TreeNode) -> List[List[int]]:
if not root:
return None
res, q = [], [root]
while q:
val, tmp = [], []
for node in q:
val.append(node.val)
if node.left:
tmp.append(node.left)
if node.right:
tmp.append(node.right)
res.append(val)
q = tmp
return res[::-1]
