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103.binary-tree-zigzag-level-order-traversal


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]
]
# Definition for a binary tree node.
# class TreeNode(object):
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None
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


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