Частота символа в списке символов

one.scala

val str = "abAsdhqfdkjggjsjfpg".toList
val str1 = str
  .groupBy(identity)                // s -> List(s,s)
  .mapValues(_.size)                // s -> 2
  //.map( e => (e._1,e._2.length))  // s -> 2
  .toList                           // (s,2)

// with tail recursion
def timesFirst(chars: List[Char]) : List[(Char,Int)] = {
  @tailrec // comiler cheks if really tailrecursive
  def timesTail(chars: List[Char], res: List[(Char,Int)])  : List[(Char,Int)] =
    chars match {
      case Nil => res      // we are done when there are no characters left
      case char :: rest => {
        // otherwise
        val newCharCount = res
          .find (_._1 == char) //check if we already have seen the character
          .map{ case (c,count) => (c,count + 1)  } // if yes, we raise take the old count and raise it by one
          .getOrElse( (char,1) ) // otherwise we count one occurrence

        // here it gets recursive
        timesTail(
          rest, // remaining Characters
          newCharCount :: res.filterNot(_._1 == char) // add the new count to list, remove old if present
        )
      }
    }
  // initial call with empty lsit to start the helper function
  timesTail(chars,List())
}

// without tail recursion
def timesSecond(chars: List[Char]): List[(Char, Int)] = {
  def incr(acc:Map[Char, Int], c:Char) = {
    val count = (acc getOrElse(c, 0)) + 1
    acc + ((c, count))
  }

  (Map[Char,Int]() /: chars)(incr).toList
}