題目：給定一個(gè)二叉樹孽尽，返回其節(jié)點(diǎn)值自底向上的層次遍歷很洋。 （即按從葉子節(jié)點(diǎn)所在層到根節(jié)點(diǎn)所在的層蜈缤，逐層從左向右遍歷）
示例 :
給定二叉樹 [3,9,20,null,null,15,7]
3
/ \
9 20
/ \
15 7
返回其自底向上的層次遍歷為：
[
[15,7],
[9,20],
[3]
]
解答：
# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None
class Solution:
def levelOrderBottom(self, root: TreeNode) -> List[List[int]]:
if root == None:
return []
stack = [root]
res = []
while len(stack) != 0:
tmp = []
res_each = []
for i in stack:
res_each.append(i.val)
if i.left != None:
tmp.append(i.left)
if i.right != None:
tmp.append(i.right)
stack = tmp
res.insert(0,res_each)
return res
