Top n items in a List ( including duplicates )

Question

Trying to find an efficient way to obtain the top N items in a very large list, possibly containing duplicates.

I first tried sorting & slicing, which works. But

Answer 1

I wanted a version that was polymorphic, and also allowed to compose using a single iterator. For instance, what if you wanted the top largest and smallest elements when reading from a file? Here is what I came up with:

    import util.Sorting.quickSort

    class TopNSet[T](n:Int) (implicit ev: Ordering[T], ev2: ClassManifest[T]){
      val ss = new Array[T](n)
      var len = 0

      def tryElement(el:T) = {
        if(len < n-1){
          ss(len) = el
          len += 1
        }
         else if(len == n-1){
          ss(len) = el
          len = n
          quickSort(ss)
        }
        else if(ev.gt(el, ss(0))){
          ss(0) = el
          quickSort(ss)
        }
      }
      def getTop() = {
        ss.slice(0,len)
      }
    }

Evaluating compared to the accepted answer:

val myInts = Array.fill(100000000)(util.Random.nextInt)
time(topNs(myInts,100)
//Elapsed time 3006.05485 msecs
val myTopSet = new TopNSet[In](100)
time(myInts.foreach(myTopSet.tryElement(_)))
//Elapsed time 4334.888546 msecs

So, not much slower, and certainly a lot more flexible