solution.py

# Definition for a binary tree node.
from typing import List, Optional


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]:
        # solution 1:
        # res = []

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

        # solution 2:
        res = []
        stack = []
        curr = root
        while curr or stack:
            while curr:
                stack.append(curr)
                curr = curr.left
            curr = stack.pop()
            res.append(curr.val)
            curr = curr.right
        return res