528 // Recursion

   529 //===========

   530 

   531 /* a, b, c

   532 

   533 aa         aaa

   534 ab         baa 

   535 ac         caa 

   536 ba  =>     ......

   537 bb

   538 bc

   539 ca

   540 cb

   541 cc

   542 

   543 */

   544 

   545 def perms(cs: List[Char], l: Int) : List[String] = {

   546   if (l == 0) List("")

   547   else for (c <- cs; s <- perms(cs, l - 1)) yield s"$c$s"

   548 }

   549 

   550 perms("abc".toList, 2)

   551 

   552 def move(from: Char, to: Char) =

   553   println(s"Move disc from $from to $to!")

   554 

   555 def hanoi(n: Int, from: Char, via: Char, to: Char) : Unit = {

   556   if (n == 0) ()

   557   else {

   558     hanoi(n - 1, from, to, via)

   559     move(from, to)

   560     hanoi(n - 1, via, from, to)

   561   }

   562 } 

   563 

   564 hanoi(40, 'A', 'B', 'C')

   565 

   566