refactoring: case(for permutation and combination)

src/main/c/Calculator/calculator.c

@@ -107,6 +107,14 @@ double combination(int n, int r);
         case 17:
             return tan(num1 * M_PI / 180.0);
               
+        case 18:
+            return permutation((int)num1, (int)num2);
+        case 19:
+            return combination((int)num1, (int)num2);
+        default:
+            return 0.0;
+    }
+}