Binary Tree Inorder Traversal

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

Output: [1,3,2]

The tree has root 1, with right child 2, which has left child 3. Inorder visits: 1 (no left child), then 3 (left of 2), then 2.

Input: root = [1,2,3,4,5,null,8,null,null,6,7,9]

Output: [4,2,6,5,7,1,3,9,8]

Full traversal of a more complex tree, visiting left subtree first, then root, then right subtree at each level.

Input: root = []

Output: []

An empty tree returns an empty list.

Input: root = [1]

Output: [1]

A single node tree returns just that node's value.

Problem