solution in scala

def biggerIsGreater(w: String): String = {
  var i = w.length - 1
  val charMap = mutable.Map[Char, Int]()
  var replacePos: Option[Int] = None
  while(i >= 0 && replacePos.isEmpty){
    charMap(w(i)) = i
    val nextLargest = charMap
      .filter { case (k, _) => k > w(i) }
      .minByOption { case (k, _) => k }
    replacePos = nextLargest match{
      case None => {
        i = i - 1
        None
      }
      case Some(s) => Some(charMap(s._1))
    }
  }
  replacePos match{
    case None => "no answer"
    case Some(s) => {
      val switched = w.updated(s, w(i)).updated(i, w(s))
      // sort after the initial switch to efficiently arrive at the smallest possible substring
      val (prefix, suffix) = switched.splitAt(i + 1)
      val sortedSuffix = suffix.sorted
      prefix + sortedSuffix
    }
  }
}