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