103. 二叉树的锯齿形层次遍历

https://leetcode-cn.com/problems/binary-tree-zigzag-level-order-traversal/

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class Solution:
    def zigzagLevelOrder(self, root: TreeNode) -> List[List[int]]:
        res = []

        def helper(root, depth):
            if not root:
                return

            if len(res) == depth:
                res.append([])
            if depth % 2 == 0:
                res[depth].append(root.val)
            else:
                res[depth].insert(0, root.val)

            helper(root.left, depth + 1)
            helper(root.right, depth + 1)

        helper(root, 0)
        return res