Problem Statement
Given the root of a binary tree, return the inorder traversal of its nodes' values. (link)
Example 1:
Input: root = [1,null,2,3]
Output: [1,3,2]
Explanation:
Example 2:
Input: root = [1,2,3,4,5,null,8,null,null,6,7,9]
Output: [4,2,...
