Binary Tree Inorder Traversal

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] **Explanation:** **Example 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] **Explanation:** **Example 3:** **Input:** root = [] **Output:** [] **Example 4:** **Input:** root = [1] **Output:** [1]   **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?