SICP

Permutations.scala

object Permutations {

  def permutations[A](s: Set[A]): List[List[A]] = {
    if (s.size == 1) List(s.toList)
    else s.flatMap(x => permutations(s - x).map(p => List(x) ::: p)).toList
  }

  def main(args: Array[String]): Unit = {
    println(permutations(Set(1, 2, 3)).mkString("\n"))
  }
}