Given the root
of a binary tree, return the inorder traversal of its nodes' values.
Input: root = [1,null,2,3]
Output: [1,3,2]
Input: root = []
Output: []
Input: root = [1]
Output: [1]
Input: root = [1,2]
Output: [2,1]
Input: root = [1,null,2]
Output: [1,2]