isSorted

   @tailrec
  final def isSorted[A](as: Array[A], gt: (A, A) => Boolean): Boolean = {
    as match {
      case Array() => true
      case Array(a) => true
      case Array(a, b) => !gt(a, b)
      case _ => if (gt(as(0), as(1))) false else isSorted(as.tail, gt)
    }
  }


  // Exercise 2: Implement a polymorphic function to check whether
  // an `Array[A]` is sorted
  def isSorted[A](as: Array[A], gt: (A,A) => Boolean): Boolean = {
    @annotation.tailrec
    def go(i: Int, prev: A): Boolean =
      if (i == as.length) true
      else if (gt(as(i), prev)) go(i + 1, as(i))
      else false
    if (as.length == 0) true
    else go(1, as(0))
  }