Restore DigitalRoot legacy API compatibility

src/main/java/kyu6/DigitalRoot.java

@@ -28,6 +28,18 @@ public static int digitalRoot(int n) {
         return result;
     }
 
+    /**
+     * Retains compatibility with the original Codewars API.
+     *
+     * @param n a non-negative integer
+     * @return the digital root of {@code n}
+     * @deprecated use {@link #digitalRoot(int)} instead
+     */
+    @Deprecated
+    public static int digital_root(int n) {
+        return digitalRoot(n);
+    }
+
     /**
      * Digital root is the recursive sum of all the digits in a number.
      * Given n, take the sum of the digits of n. If that value has more than one

src/test/java/kyu6/DigitalRootTest.java

@@ -21,5 +21,6 @@
     void shouldReduceNumberToSingleDigit(int value, int expected) {
         assertEquals(expected, DigitalRoot.digitalRoot(value));
         assertEquals(expected, DigitalRoot.digitalRootRecursiveStream(value));
+        assertEquals(expected, DigitalRoot.digital_root(value));
     }
 }