Missing persist in mllib.PrefixSpan.findFrequentItems()

  private[fpm] def findFrequentItems[Item: ClassTag](
      data: RDD[Array[Array[Item]]],
      minCount: Long): Array[Item] = {

    data.flatMap { itemsets =>
      val uniqItems = mutable.Set.empty[Item]
      itemsets.foreach(set => uniqItems ++= set)
      uniqItems.toIterator.map((_, 1L))
    }.reduceByKey(_ + _).filter { case (_, count) =>
      count >= minCount
    }.sortBy(-_._2).map(_._1).collect()
  }