계승

int factorial(int n) => factorialCal(n);

int factorialCal(int num){  
  if(num<=1) { 
        return 1;  
  } else{  
      return num*factorial(num-1);
 }  
}

int factorial(int n) {
  return List.generate(n, (i) => i + 1).fold(1, (int a, b) => a * b);
}

int factorial(int n) => (n == 0) ? 1 : n * factorial(n - 1);