Given a binary tree, return the zigzag level order traversal of its nodes' values. (ie, from left to right, then right to left for the next level and alternate between).
For example:
Given binary tree [3,9,20,null,null,15,7],
3
/ \
9 20
/ \
15 7
return its zigzag level order traversal as:
[
[3],
[20,9],
[15,7]
]
from collections import deque
class Solution(object):
def zigzagLevelOrder(self, root):
"""
:type root: TreeNode
:rtype: List[List[int]]
"""
stack = deque([root])
ans = []
odd = True
while stack:
level = []
for k in xrange(0, len(stack)):
top = stack.popleft()
if top is None:
continue
level.append(top.val)
stack.append(top.left)
stack.append(top.right)
if level:
if odd:
ans.append(level)
else:
ans.append(level[::-1])
odd = not odd
return ans