Solution.py

# 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 []
        left, ans = False, []
        q = deque([root])
        while q:
            n = len(q)
            t = []
            for _ in range(n):
                node = q.popleft()
                t.append(node.val)
                if node.left:
                    q.append(node.left)
                if node.right:
                    q.append(node.right)
            if left:
                t.reverse()
            ans.append(t)
            left = not left
        return ans