给定一个二叉树,返回其节点值自底向上的层序遍历。 (即按从叶子节点所在层到根节点所在的层,逐层从左向右遍历)
例如:
给定二叉树 [3,9,20,null,null,15,7],
3
/ \
9 20
/ \
15 7
返回其自底向上的层序遍历为:
[
[15,7],
[9,20],
[3]
]
广度优先遍历,再将结果列表反向输出。
# Definition for a binary tree node.
# class TreeNode(object):
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None
class Solution(object):
def levelOrderBottom(self, root):
"""
:type root: TreeNode
:rtype: List[List[int]]
"""
if root is None:
return []
que = []
que.append(root)
res_all = []
while que:
res_level = []
for _ in range(len(que)):
cur_node = que.pop(0)
if cur_node:
res_level.append(cur_node.val)
if cur_node.left:
que.append(cur_node.left)
if cur_node.right:
que.append(cur_node.right)
if res_level:
res_all.append(res_level)
# res_all.reverse()
# return res_all
return res_all[::-1]