Scalania15.scala

  def filterNot[T](ts: Seq[T])(p: T => Boolean): Seq[T] =
    (ts :\ Seq[T]())((t, a) => if (!p(t)) t +: a else a)

  @tailrec  
  def drop[T](ts: Seq[T])(n : Int): Seq[T] = (n, ts) match {
    case (0, _) => ts
    case (n, _ +: tts) if n > 0 => drop(tts)(n - 1)
    case _ => Seq()
  }

  def duplicate[T](ts: Seq[T]): Seq[T] = 
    ts.flatMap(x => Seq(x, x))

  def duplicate[T](ts: Seq[T]): Seq[T] = {
    @tailrec
    def go(ts: Seq[T], acc: Seq[T]): Seq[T] = ts match {
      case Seq() => acc
      case head +: tail => go(tail, head +: head +: acc)
    }
    go(ts.reverse, Seq())
  }

  def duplicate[T](ts: Seq[T]): Seq[T] = 
    ts.flatMap(x => (1 to 2).flatMap(n => List(x)))