How to find the most common elements of an array?

You can capture the max frequency similar to what you've come up with via groupBy/mapValues/maxBy, then apply collect to the frequency Map to obtain the list of elements with the max frequency, as shown below:

val list = List(5, 4, 3, 2, 4, 5, 1, 6, 1, 2, 5, 4)

val freqMap = list.groupBy(identity).mapValues(_.size)

val maxFreq = freqMap.maxBy(_._2)._2

freqMap.collect{ case (elem, freq) if freq == maxFreq => elem }
// res1: scala.collection.immutable.Iterable[Int] = List(5, 4)

An alternative to Leo's answer based on Scala 2.13 which allows us to additionally handle empty lists:

val list = List(5, 4, 3, 2, 4, 5, 1, 6, 1, 2, 5, 4)
val freqMap = list.groupMapReduce(identity)(_ => 1)(_+_)
// HashMap(5 -> 3, 1 -> 2, 6 -> 1, 2 -> 2, 3 -> 1, 4 -> 3)
val maxFreq = freqMap.maxByOption(_._2).map(_._2)
// Option[Int] = Some(3)
maxFreq.map(max => freqMap.collect { case (x, f) if f == max => x }).getOrElse(Nil)
// List(5, 4)

This:

• Creates a frequency map with Scala 2.13's new groupMapReduce:

  • groups items (group part of groupMapReduce)

  • maps each grouped value occurrence to 1 (map part of groupMapReduce)

  • reduces values within a group of values (_ + _) by summing them (reduce part of groupMapReduce).

• Retrieves an optional maximum frequency using Scala 2.13's new maxByOption in order to avoid an exception on empty input lists.

• And finally maps the optional max frequency in order to collect only items with the max frequency and otherwise return an empty list.