String K-Reduction implemented in Scala

gistfile1.txt

val input = "abccaaaba" // cccaaaba -> ccbaaba -> caaaba -> baaba -> caba -> bba -> bc -> a

val distinctChars = input.distinct.sorted.toList

@tailrec
def reduceString(acc: List[Char], seq: List[Char]): List[Char] = seq match {
  case Nil =>
    if (acc.toSet.size == 1) acc
    else reduceString(List.empty[Char], acc)
  case x :: Nil =>
    reduceString(acc :+ x, List.empty[Char])
  case x :: xs =>
    if (x != xs.head)
      reduceString(
        acc,
        distinctChars.diff(
          Seq(x, xs.headOption.getOrElse(List.empty[Char]))
        ) ++ Option(xs.tail).getOrElse(List.empty[Char])
      )
    else
      reduceString(x :: acc, xs)
}

println(reduceString(List.empty[Char], input.toCharArray.toList))

Given a string consisting of a,b and c's, we can perform the following operation: Take any two adjacent distinct characters and replace it with the third character. For example, if 'a' and 'c' are adjacent, they can replaced with 'b'.

What is the smallest string which can result by applying this operation repeatedly