https://www.hackerrank.com/challenges/filter-elements

FilterElements.scala

/**
  * Created by janisrumnieks on 07/03/2016.
  * Hackerrank -> Functional Programming -> Recursion -> Filter Elements (10)
  * https://www.hackerrank.com/challenges/filter-elements
  */
object FilterElements /* rename this to Solution for Hackerrank */ extends App {

  // helper functions
  def countElementsR(occurrences: Int, list: List[Int]):List[Int] = list match {
    case i::rest => if (rest.count(_ == i) >= occurrences-1)
                      i :: countElementsR(occurrences, rest.filter( _ != i))
                    else
                      countElementsR(occurrences, rest.filter( _ != i))
    case Nil => Nil
  }

  // two functions instead of a one - looks like a fail to me...
  def countElements (occurrences: Int, list: List[Int]): List[Int] = {
    val result = countElementsR(occurrences, list)
    if (result == Nil) List(-1) else result
  }


  // 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) => countElements(occurrences, list) }
    .map { l => println( l.mkString(" ") ) }

}

You could use Specs2 https://etorreborre.github.io/specs2/ or ScalaTest http://www.scalatest.org/ (and even ScalaCheck https://www.scalacheck.org/) for tests. They are all really nice.

Import io.StdIn.readLine or at least io.StdIn.

IMHO you can share the countElementsR(occurrences, rest.filter( _ != i)) somehow instead of copy-pasting it.

If you hate the two functions, you could also move the if (result == Nil) List(-1) else result part below into the .map portion, as really it is part of output formatting.

I dislike the formatting at places, like lack of spaces around - in occurrences-1, how lines 10-13 are formatted.