'''
Binary Tree Zigzag Level Order Traversal

Solution
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, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right


class Solution:
    def zigzagLevelOrder(self, root: TreeNode) -> List[List[int]]:
        if root is None:
            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)
            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

'''
Run Code Status: Finished

×
Run Code Result:

Your input

[3,9,20,null,null,15,7]
Your answer

[[3],[20,9],[15,7]]
Expected answer

[[3],[20,9],[15,7]]
'''
