makotan

    val map = List(1,2,3,4,1,2).foldLeft(Map[Int,Int]().withDefaultValue(0)){
      (map,key) =>
        map + (key -> (map(key) + 1))
    }.foldLeft(List[Int](),0) {
      case ((set,c),(k,v)) if(c < v) => (List(k),v)
      case ((set,c),(k,v)) if(c == v) => ((k +: set),v)
      case ((set,c),(k,v)) => (set,c)
    } _1