99 Scala (problems 8 - 12) answers

99problems_8_12.scala

//p08
def compress[A](xs: List[A]): List[A] =
  xs match {
    case x :: xs if (!xs.contains(x)) => x :: compress(xs)
    case x :: xs => compress(xs)
    case Nil => Nil
  }

def compress2[A](xs: List[A]): List[A] = {
  xs.foldRight(List.empty[A]){
    case (x,acc) => if (!acc.contains(x)) x :: acc else acc
  }
}


//
val myList = "aaabbcccccd" toList

//p09
def pack[A](l:List[A]): List[List[A]] = {
  l match {
    case x :: xs =>
      xs.foldLeft(List(List(x))){
        case (z,a) =>
          if (z.head.head == a)
            (a +: z.head) +: z.tail
          else
            List(a) +: z
      }.reverse
    case _ => Nil
  }
}

pack(myList)

//p10
def encode[A](xs: List[A]): List[(Int,A)] =
  pack(xs) map {x => (x.size,x.head) }

encode(myList)

//p11
def encodeModified[A](xs: List[A]): List[Any] =
  encode(xs) map {x => if (x._1 ==1) x._2 else x}

encodeModified(myList)

//p12
def decode[A](ls: List[(Int,A)]): List[A] =
  ls flatMap {x => List.fill(x._1)(x._2)}

decode(encode(myList))