Solution.scala

/** https://www.hackerrank.com/challenges/filter-elements */
object Solution extends App {
  def filterElements(list: List[Int], times: Int, processed: Set[Int] = Set.empty): List[Int] = {
    if (list.isEmpty) Nil
    else {
      val elem = list.head
      val x = if (processed.contains(elem)) {
        Nil // already seen
      } else {
        if (list.count(_ == elem) >= times) List(elem) // right count
        else Nil // wrong count
      }

      x ::: filterElements(list.tail, times, processed + elem)
    }
  }

  val in = io.StdIn
  val testCases = in.readInt()
  Range(0, testCases)
    .map { _ =>  ( in.readLine().split(' ')(1).toInt, in.readLine().split(' ').map(_.toInt).toList )  }
    .map { case (occurrences, list) => filterElements(list, occurrences) }
    .map { l => if (l.isEmpty) "-1" else l.mkString(" ") }
    .foreach(println)
}

SolutionSpec.scala

import org.scalatest.{FlatSpec, Matchers}

class SolutionSpec extends FlatSpec with Matchers {
  it should "pass test case 1" in {
    Solution.filterElements(List(4, 5, 2, 5, 4, 3, 1, 3, 4), 2) shouldEqual List(4, 5, 3)
  }

  it should "pass test case 2" in {
    Solution.filterElements(List(4, 5, 2, 5, 4, 3, 1, 3, 4), 4) shouldEqual Nil
  }

  it should "pass test case 3" in {
    Solution.filterElements(List(5, 4, 3, 2, 1, 1, 2, 3, 4, 5), 2) shouldEqual List(5, 4, 3, 2, 1)
  }
}