Ninety-Nine Scala Problems: Problem 08 - Eliminate consecutive duplicates of list elements.

Problem08.scala

// http://aperiodic.net/phil/scala/s-99/

// P08 (**) Eliminate consecutive duplicates of list elements.
// If a list contains repeated elements they should be replaced with a single copy of the element. The order of the elements should not be changed.
// Example:

// scala> compress(List('a, 'a, 'a, 'a, 'b, 'c, 'c, 'a, 'a, 'd, 'e, 'e, 'e, 'e))
// res0: List[Symbol] = List('a, 'b, 'c, 'a, 'd, 'e)

object Problem08 {
  def main(args: Array[String]) {
    val values = List('a, 'a, 'a, 'a, 'b, 'c, 'c, 'a, 'a, 'd, 'e, 'e, 'e, 'e)
    println(compress(values))
  }

  // def compress[T](values: List[T]) : List[T] = 
  //   values match {
  //     case Nil                      => Nil
  //     case x :: y :: xs if (x == y) => compress(y :: xs) 
  //     case x :: xs                  => List(x) ::: compress(xs)
  //   }

  def compress[T](values: List[T]) : List[T] = 
    compressTail(Nil, values)

  def compressTail[T](seen: List[T], remaining: List[T]) : List[T] = 
    remaining match {
      case Nil                      => seen
      case x :: y :: xs if (x == y) => compressTail(seen, y :: xs)
      case x :: xs                  => compressTail(seen ::: List(x), xs)
    }
}