Binary Tree Zigzag Level Order Traversal - Python Leetcode

 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:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
from collections import deque

class Solution:
    def zigzagLevelOrder(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        if not root:
            return []
        
        s1 = [root]
        s2 = []
        
        level =[]
        result = []
        
        while s1 or s2:
            while s1:
                root = s1.pop()
                level.append(root.val)
                if root.left:
                    s2.append(root.left)
                if root.right:
                    s2.append(root.right)
            if level:
                result.append(level)
            level = []
            while s2:
                root = s2.pop()
                level.append(root.val)
                if root.right:
                    s1.append(root.right)
                if root.left:
                    s1.append(root.left)
            if level:
                result.append(level)
            level = []
        return result
                
            
    TC : O(n)

SC:O(n)