Permutations

// permutations:
object Main extends App {
  def f(base: List[Int], acc: List[Int]): List[List[Int]] = {
    if (base.size == acc.size) List(acc)                  // if accumulator contains all elements => return
    else base filter(!acc.contains(_)) flatMap { w =>     // otherwise fall into recursion
      f(base, w :: acc)
    }
  }

  f(List(1, 2, 3), Nil) foreach println
}

// Output:
// List(3, 2, 1)
// List(2, 3, 1)
// List(3, 1, 2)
// List(1, 3, 2)
// List(2, 1, 3)
// List(1, 2, 3)