ref: update implementation

- 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

Author: sozelfist

src/main/java/com/thealgorithms/datastructures/trees/SplayTree.java

@@ -47,6 +47,15 @@ public SplayTree() {
         root = null;
     }
 
+    /**
+     * Checks if the tree is empty.
+     *
+     * @return True if the tree is empty, otherwise false.
+     */
+    public boolean isEmpty() {
+        return root == null;
+    }
+
     /**
      * Zig operation.
      *
@@ -202,7 +211,7 @@ public boolean search(int key) {
      * @throws IllegalArgumentException If the tree is empty.
      */
     public void delete(int key) {
-        if (root == null) {
+        if (isEmpty()) {
             throw new IllegalArgumentException("Cannot delete from an empty tree");
         }
 
@@ -243,6 +252,8 @@ public List<Integer> traverse(TraverseOrder traverseOrder) {
         case POST_ORDER:
             postOrderRec(root, result);
             break;
+        default:
+            throw new IllegalArgumentException("Invalid traversal order: " + traverseOrder);
         }
         return result;
     }