scalania19.scala

  def mostCommon(l: ParSeq[Int]): Int = {
    type Freq = Map[Int, Int]
    def addToFreq(freq: Freq, v: Int, f: Int) =
      freq.updated(v, freq.getOrElse(v, 0) + f)
    def mapFreq(freq: Freq, v: Int)  =
      addToFreq(freq, v, 1)
    def reduceFreq(freq1: Freq, freq2: Freq) =
      freq1.foldLeft(freq2) { case (freq, (v, f)) => addToFreq(freq, v, f) }
    val freq = l.aggregate(Map[Int, Int]())(mapFreq, reduceFreq)
    freq.toSeq.maxBy(_._2)._1
  }