Finding second highest frequency of characters in String

gistfile1.scala

package puzzlers

import scala.collection.mutable.{Map =>  MMap}

object HighestFrequencyPuzzle {

  def stringToCharFrequencyMap(source: String): MMap[Char, Int] = {
    source.filter(c => c!=' ').foldLeft(MMap[Char, Int]())((map, char) => {
      map.update(char, map.getOrElseUpdate(char, 0) + 1)
      map 
    })
  }
  
  def getNthHighestFrequency(source: String, n: Int): (Char, Int) = 
     stringToCharFrequencyMap(source).toList.sortBy(_._2).reverse.drop(n -1).take(1)(0)
  
  def main(args: Array[String]): Unit = {
    val highest = getNthHighestFrequency("abecbb cdcbef", 1)
    println(highest)
    val secondHighest = getNthHighestFrequency("abecbb cdcbef", 2)
    println(secondHighest)
  }

}

/** output
(b,4)
(c,3)
**/