"""
Given the root of a binary tree, return the inorder traversal of its nodes' values.

Example 1:
Input: root = [1,null,2,3]
Output: [1,3,2]

Example 2:
Input: root = []
Output: []

Example 3:
Input: root = [1]
Output: [1]

Constraints:
The number of nodes in the tree is in the range [0, 100].
-100 <= Node.val <= 100

Follow up: Recursive solution is trivial, could you do it iteratively?
"""
from typing import Optional, List


class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right


class BinaryTreeInorderTraversal:
    def inorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
        res, stack, cur = [], [], root
        if not root:
            return res
        while cur or stack:
            if cur:
                stack.append(cur)
                cur = cur.left
            else:
                cur = stack.pop()
                res.append(cur.val)
                cur = cur.right
        return res
