[LC] 103. Binary Tree Zigzag Level Order Traversal

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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] ,

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