from typing import List, Optional


# 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 inorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
        if root is None:
            return []
        if root.left is not None:
            return (
                self.inorderTraversal(root.left)
                + [root.val]
                + self.inorderTraversal(root.right)
            )
        elif root.right is not None:
            return [root.val] + self.inorderTraversal(root.right)
        else:
            return [root.val]