Scalania16.scala

  def drop[T](n: Int, ts: Seq[T]): Seq[T] = {
    @tailrec
    def go(i: Int, ts: Seq[T], acc: Seq[T]): Seq[T] = (i, ts) match {
      case (`n`, _ +: t) => go(0, t, acc)
      case (_, h +: t) => go(i + 1, t, h +: acc)
      case _ => acc.reverse
    }
    go(0, ts, Seq())
  } 

  def drop[T](n: Int, ts: Seq[T]): Seq[T] = {
    @tailrec
    def go(i: Int, ts: Seq[T], acc: Seq[T]): Seq[T] = (i, ts) match {
      case (i, _ +: t) if (i == n) => go(0, t, acc)
      case (_, h +: t) => go(i + 1, t, h +: acc)
      case _ => acc.reverse
    }
    go(0, ts, Seq())
  }

  def takeWhile[T](ts: Seq[T])(p: T => Boolean): Seq[T] = {
    @tailrec
    def go(ts: Seq[T], acc: Seq[T]): Seq[T] = ts match {
      case h +: t if p(h) => go(t, h +: acc)
      case _ => acc.reverse
    }
    go(ts, Seq())
  }

  def isPalindrome1[T](ts: Seq[T]): Boolean =
    ts == ts.reverse

  def isPalindrome[T](ts: Seq[T]): Boolean =
    ts.zip(ts.reverse) forall { case (a, b) => a == b }