Binary Tree: Inorder Traversal

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

class TreeNode:
    def __init__(self, x: int) -> None:
        self.val = x
        self.left = None
        self.right = None

class Solution:
    def traversal(self, root: 'TreeNode', result=None) -> List[List[int]]:
        """
                    0
                 /     \    
                1        2
              /   \
            3       4
                  /   \
                6       9

        In-order...: [3, 1, 6, 4, 9, 0, 2]

        T: O(N)
        S: O(N) worst case; O(log N) average
        """

        if result is None:
            result = []

        if not root:
            return []
        
        self.traversal(root.left, result=result)
        out.append(root.val)
        self.traversal(root.right, result=result)

        return result

class Solution:
    def traversal(self, root: 'TreeNode')->List[List[int]]:
        """
                    0
                 /     \    
                1        2
              /   \
            3       4
                  /   \
                6       9

        In-order...: [3, 1, 6, 4, 9, 0, 2] 

        T: O(N)
        S: O(N)
        """
        if not root:
            return []

        stack = []
        result = []
        node = root

        while node or stack:
            # Traverse down to bottom of the left branch
            while node:
                stack.append(node)
                node = node.left
            
            # Visit root and then go right
            node = stack.pop()
            result.append(node.val)
            node = node.right
            
        return result