S-99 P01-P10

p01-p10.scala

// P01
def last[A](l: List[A]): A = l match{
  case n :: Nil => n
  case n :: next => last(next)
  case _ => throw new NoSuchElementException
}

// P02
def penultimate[A](l: List[A]): A = l match{
  case n :: m :: Nil => n
  case n :: next => penultimate(next)
  case _ => throw new NoSuchElementException
}

// P03
def nth[A](c: Int, l: Seq[A]): A = (c, l) match{
  case (0, n :: _) => n
  case (n, _ :: next) => nth(n-1, next)
  case (_, _) => throw new NoSuchElementException
}

// P04
def length[A](l: Seq[A]): Int = l match{
  case Nil => 0
  case _ :: next => 1 + length(next)
}

// P05
def reverse[A](l: Seq[A]): Seq[A] = l match {
  case Nil => Nil
  case n :: next => reverse(next) :+ n
}

// P06
def isPalindrome[A](l: Seq[A]): Boolean = l == l.reverse

// P07
def flatten(ls: List[Any]): List[Any] = ls flatMap{
  case ms: List[_] => flatten(ms)
  case e => List(e)
}

// P08
def compress[A](ls: Seq[A]): Seq[A] = ls match{
  case Nil => Nil
  case n :: tail => n +: compress(tail.dropWhile(_ == n))
}

// P09
def pack[A](ls: Seq[A]): Seq[Seq[A]] = {
  if (ls.isEmpty) Nil
  else{
    val (packed, next) = ls span(ls.head == _)
    packed +: pack(next)
  }
}    

// P10
def encode[A](ls: Seq[A]): Seq[(Int, A)] = pack(ls) map (n => (n.length, n.head))