binary_tree_inorder_traversal.py

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]:
        result = []

        def inorder(root):
            if not root:
                return []

            inorder(root.left)
            result.append(root.val)
            inorder(root.right)

        inorder(root)

        return result