def gemStones(stones: List[String]): Int = {
  @tailrec
  def isIn(cs: List[Char], c: Char): Boolean = {
    cs match {
      case Nil => false
      case h :: t => if (h == c) true else isIn(t, c)
    }
  }

  @tailrec
  def unique(cs: List[Char], acc: List[Char]): List[Char] = {
    cs match {
      case Nil => acc
      case h :: t => if (h != ' ' && !isIn(acc, h)) unique(t, h :: acc) else unique(t, acc)
    }
  }

  val sortedStones = stones.map(s => unique(s.toList, Nil).sorted)
  (for {
    s <- sortedStones
    c <- s
  } yield (c, sortedStones.forall(isIn(_, c))))
    .filter { case (_, present) => present }
    .groupBy { case (char, _) => char }
    .keys.toList.length
}