给定一个二叉树,返回其节点值的锯齿形层次遍历。(即先从左往右,再从右往左进行下一层遍历,以此类推,层与层之间交替进行)。
例如:
给定二叉树 [3,9,20,null,null,15,7]
,
3 / \ 9 20 / \ 15 7
返回锯齿形层次遍历如下:
[ [3], [20,9], [15,7] ]
# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None
class Solution:
def zigzagLevelOrder(self, root: TreeNode) -> List[List[int]]:
#BFS
if not root:
return []
res = []
que = [root]
flag = -1
while que:
flag = -flag
val = []
q = []
for i in que:
val.append(i.val)
if i.left:
q.append(i.left)
if i.right:
q.append(i.right)
res.append(val[::flag])
que = q
return res