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

class Solution(object):
    def zigzagLevelOrder(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        from collections import deque
        res = []
        if root is None:
            return res
        is_even = False
        my_deque = deque()
        my_deque.append(root)
        while my_deque:
            size = len(my_deque)
            lst = []
            for _ in range(size):
                if is_even:
                    cur = my_deque.pop()
                    if cur.right:
                        my_deque.appendleft(cur.right)
                    if cur.left:
                        my_deque.appendleft(cur.left)
                else:
                    cur = my_deque.popleft()
                    if cur.left:
                        my_deque.append(cur.left)
                    if cur.right:
                        my_deque.append(cur.right)
                lst.append(cur.val)
            res.append(lst)
            is_even = not is_even
        return res