void setup() {
   int x = 5;
   println(x + "! = " + factorialR(x));
   
} // setup()

long factorial (long n) {
   long result = 1;
   for (long i = n; i > 0; i--) {
      result *= i; 
   }
   return result;
} // factorial()

int factorialR(int n) {
   if (n == 0) {
      return 1;
   }
   else {
      return n * factorialR(n - 1);
   }
}