Inorder - RECURSION

    func inorderTraversal(_ root: TreeNode?) -> [Int] {
        guard let root = root else { return []}

        var res = [Int]()
        inorder(&res, root)
        return res
    }
    func inorder(_ res: inout [Int], _ node: TreeNode?){
        guard let node = node else {return }
        inorder(&res, node.left )
        res.append(node.val)
        inorder(&res, node.right )
    }