factorial static method

Implementation

static factorial(int n) {
  var a = [1];
  return (int n) {
    var s = 1;
    if (a[n] > 0) {
      return a[n];
    }
    for (int i = n; i > 1; i--) {
      s *= i;
    }
    a[n] = s;
    return s;
  };
}