class Solution:
    def inorderTraversal(self, root: TreeNode) -> List[int]:
        if not root:
            return []
        stack = [root]
        res = []
        while len(stack) > 0:
            curr_node = stack.pop()
            if len(stack) > 0 and stack[len(stack) - 1] is None:
                res.append(curr_node.val)
                stack.pop()
            else:
                if curr_node.right:
                    stack.append(curr_node.right)
                stack.append(None)
                stack.append(curr_node)

                if curr_node.left:
                    stack.append(curr_node.left)
        return res