# Question

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]
]


# Solution

# 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):
"""
:type root: TreeNode
:rtype: List[List[int]]
"""
res = []
if root is not None:
level = 1
nodes = list()
nodes.append(root)
while True:
vals = [n.val for n in nodes]
res.append(vals)
new_nodes = list()

if level % 2 == 1:
for i in range(len(nodes) - 1, -1, -1):
if nodes[i].right is not None:
new_nodes.append(nodes[i].right)
if nodes[i].left is not None:
new_nodes.append(nodes[i].left)
else:
for i in range(len(nodes) - 1, -1, -1):
if nodes[i].left is not None:
new_nodes.append(nodes[i].left)
if nodes[i].right is not None:
new_nodes.append(nodes[i].right)

nodes = new_nodes
level += 1

if len(nodes) == 0:
break

return res