success!

FilterElements.scala

object FilterElements4 /* rename this to Solution for Hackerrank */ extends App {

  // helper functions
  def occurringElements(occur: Int, list: List[Int]):Set[Int] = {
    list.groupBy(identity).mapValues( _.size ).filter{ case(key,value) => value >= occur }.keys.toSet
  }

  def filterListByOccurrences(list: List[Int], occurringElements: Set[Int]): List[Int] = {
    if (list == Nil || occurringElements.isEmpty) Nil
    else {
      val (l :: rest) = list
      if ( occurringElements contains l ) l :: filterListByOccurrences( rest, occurringElements - l )
      else filterListByOccurrences( rest, occurringElements )
    }
  }

  // MAIN
  val testCases = io.StdIn.readInt()

  Range(0,testCases)
    .map { _ =>  ( io.StdIn.readLine().split(' ')(1).toInt , io.StdIn.readLine().split(' ').toList.map(_.toInt) )  }
    .map { case (occurrences, list) => ( list, occurringElements(occurrences, list) ) }
    .map { case (list, occurringElements) =>  filterListByOccurrences(list, occurringElements)}
    .map { l => if (l == Nil) List(-1) else l }
    .map { l => println( l.mkString(" ") ) }

}