http://stackoverflow.com/posts/31974656

GroupTest2.scala

package arsng

import scala.collection.mutable
import scala.collection.mutable.ListBuffer
import scala.util.Random

object GroupTest2 extends App {

  val r = new Random(1234)
  val myList = (0 to 1000000).map(_ => r.nextInt(10000)).toList
  val thresh = 3

  def measure(iterations:Int)(x: => Unit) = {
    val times = new ListBuffer[Long]
    for (i <- 0 to iterations) {
      val time = System.currentTimeMillis()
      x
      if (i > iterations / 3) {
        times += (System.currentTimeMillis() - time)
      }
    }
    times.sum / times.size
  }

  def authSolve = {
    myList.foldLeft(Map[Int,Int]()){case(m, i) => m + (i -> (m.getOrElse(i, 0) + 1))}.filter(_._2 >= thresh).keys
  }

  def ixxSolve = {
    val tuple = (Map[Int, Int](), List[Int]())
    val res = myList.foldLeft(tuple) {
      case ((m, s), i) =>
        val count = m.getOrElse(i, 0) + 1
        (m + (i -> count), if (count == thresh) i :: s else s)
    }
    res._2
  }

  def ixxSolve2 = {
    val tuple = (Map[Int, Int](), List[Int]())
    val res = myList.foldLeft(tuple) {
      case ((m, s), i) =>
        val count = m.getOrElse(i, 0) + 1
        (if (count <= 3) m + (i -> count) else m, if (count == thresh) i :: s else s)
    }
    res._2
  }

  def mySolve = {
    val map = new mutable.HashMap[Int, Int]()
    val res = new ListBuffer[Int]
    myList.foreach {
      i =>
        val c = map.getOrElse(i, 0) + 1
        if (c == thresh) {
          res += i
        }
        if (c <= thresh) {
          map(i) = c
        }
    }
    res
  }

  println("Authors solution")
  println(measure(20) {
    authSolve
  })

  println("Ixx solution: ")
  println(measure(20) {
    ixxSolve
  })

  println("Ixx solution2 (eliminated excessive map writes): ")
  println(measure(20) {
    ixxSolve2
  })

  println("My solution: ")
  println(measure(20) {
    mySolve
  })

  println(mySolve.toSet == ixxSolve.toSet)
  println(mySolve.toSet == authSolve.toSet)
  println(ixxSolve.toSet == authSolve.toSet)
  println(ixxSolve2.toSet == authSolve.toSet)
}