- Do not allow duplicate keys and explicitly throw an exception if one is trying to include already existing key
- Throw error on deletion if tree is empty
- Performing the splay operation before recursion, ensuring that the node to be deleted is at the root or very close to it, simplifying the deletion process.

- inner assignments should be avoided

- Add `isEmpty` method to check to check the entire tree root and continue using `root == null` checks within recursive calls
- Add default branch to switch statement

Each variable declaration must be in its own statement. [MultipleVariableDeclarations]

- Implement 3 traversal class of the `TreeTraversal` interface
- Addjust tests to better cover code.

- Make TreeTraversal interface implementations final
- Update tests