19.Answer

/*
The discussion about `map` also applies here.
*/
def filter[A](l: List[A], f: A => Boolean): List[A] =
  foldRight(l, Nil: List[A], (h, t) => if f(h) then Cons(h, t) else t)

def filter_1[A](l: List[A], f: A => Boolean): List[A] =
  foldRightViaFoldLeft(l, Nil: List[A], (h, t) => if f(h) then Cons(h, t) else t)

def filter_2[A](l: List[A], f: A => Boolean): List[A] =
  val buf = new collection.mutable.ListBuffer[A]
  def go(l: List[A]): Unit = l match
    case Nil => ()
    case Cons(h, t) => if f(h) then buf += h; go(t)
  go(l)
  List(buf.toList*) // converting from the standard Scala list to the list we've defined here