interesting that when List[Char] is replaced with String, this is dead slow

FullOfColours.scala

/**
  * Created by janisrumnieks on 09/03/2016.
  * https://www.hackerrank.com/challenges/sequence-full-of-colors
  */
case class Colours (red:Int, green:Int, yellow:Int, blue:Int) {
  def add (colour:Char) = colour match {
    case 'R' => Colours(red+1, green, yellow, blue)
    case 'G' => Colours(red, green+1, yellow, blue)
    case 'Y' => Colours(red, green, yellow+1, blue)
    case 'B' => Colours(red, green, yellow, blue+1)
  }

  def diffRG = math.abs(red-green)
  def diffYB = math.abs(yellow-blue)
}

//  R G at most 1,    Y and B at most 1
object FullOfColours3 extends App {

  def ballCheck (currentCols: Colours, balls:List[Char] /* very slow if this is String */): Boolean = {
    if ( balls.isEmpty && currentCols.diffRG == 0 && currentCols.diffYB == 0 ) true // all string is consumed
    else if ( balls.isEmpty) false
    else if ( currentCols.diffRG > 1 || currentCols.diffYB > 1 ) false // check prefix lengths
    else {
      val (b, rest) = (balls.head, balls.tail)
      ballCheck( currentCols.add(b), rest )
    }
  }

  val in = io.StdIn

  Range(0, in.readInt())
    .map ( _ => in.readLine().toList )
    .map ( balls => ballCheck( Colours(0, 0, 0, 0), balls) )
    .map ( b =>  if (b) "True" else "False" )
    .foreach(println)
}