public class Factorial {
  public static ulong iterative ( ulong n ) {
    ulong total = 1 ;
    for ( ulong i = 2 ; i <= n ; ++i ) { total *= i ; }
    return total ;
  }
  public static ulong recursive ( ulong n ) {
    return n <= 1 ? 1 : n * recursive ( n - 1 ) ;
  }
  private static ulong tailRecursive_iterate ( ulong n , ulong result ) {
    return n <= 1 ? result : tailRecursive_iterate ( n - 1 , result * n ) ;
  }
  public static ulong tailRecursive ( ulong n ) {
    return n <= 1 ? 1 : tailRecursive_iterate ( n , 1 ) ;
  }
}