object Main {

  def main(args: Array[String]): Unit = {
    println(charIndex("Missisipi"))
    //var s = scala.collection.immutable.SortedSet[Int]()
  }

  def charIndex(s: String) : scala.collection.mutable.Map[Char, scala.collection.mutable.TreeSet[Int]] = {
    val m = scala.collection.mutable.Map[Char, scala.collection.mutable.TreeSet[Int]]()
    s.zipWithIndex.foreach{
      case (c, i) => m += c -> (m.getOrElse(c, scala.collection.mutable.TreeSet()) += i)
    }
    m
  }
}

case ( c, i ) =>  m += c -> ( m.getOrElse( c, scala.collection.mutable.TreeSet[Int]() ) += i)

scala.collection.mutable.TreeSet()

new TreeSet()(implicit ord: Ordering[A])