class Solution:
    def inorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
        # Iterative
        res, stack = [], []
        cur = root

        while cur or stack:
            while cur:
                stack.append(cur)
                cur = cur.left
            cur = stack.pop()
            res.append(cur.val)
            cur = cur.right
        return res

        # Recursive
        res = []

        def helper(root):
            if not root:
                return
            helper(root.left)
            res.append(root.val)
            helper(root.right)

        helper(root)
        return res