Given the root of a binary tree, return the inorder traversal of its nodes' values. Example 1: Input: root = [1,null,2,3] Output: [1,3,2] Example 2: Input: root = [] Output: [] Example 3: Input: root = [1] Output: [1] Example 4: Input: root = [1,2] Output: [2,1] Example 5: Input: root = [1,null,2] Output: [1,2] Constraints: The number of nodes in the tree is in the range [0, 100]. -100 <= Node.val <= 100 Follow up: Recursive solution is trivial, could you do it iteratively?