blue-print-solution.md

// P17 (*) Split a list into two parts.
//     The length of the first part is given.  Use a Tuple for your result.
//
//     Example:
//     scala> split(3, List('a, 'b, 'c, 'd, 'e, 'f, 'g, 'h, 'i, 'j, 'k))
//     res0: (List[Symbol], List[Symbol]) = (List('a, 'b, 'c),List('d, 'e, 'f, 'g, 'h, 'i, 'j, 'k))

object P17 {
  // Builtin.
  def splitBuiltin[A](n: Int, ls: List[A]): (List[A], List[A]) = ls.splitAt(n)

  // Simple recursion.
  def splitRecursive[A](n: Int, ls: List[A]): (List[A], List[A]) = (n, ls) match {
    case (_, Nil)       => (Nil, Nil)
    case (0, list)      => (Nil, list)
    case (n, h :: tail) => {
      val (pre, post) = splitRecursive(n - 1, tail)
      (h :: pre, post)
    }
  }

  // Tail recursive.
  def splitTailRecursive[A](n: Int, ls: List[A]): (List[A], List[A]) = {
    def splitR(curN: Int, curL: List[A], pre: List[A]): (List[A], List[A]) =
      (curN, curL) match {
        case (_, Nil)       => (pre.reverse, Nil)
        case (0, list)      => (pre.reverse, list)
        case (n, h :: tail) => splitR(n - 1, tail, h :: pre)
      }
    splitR(n, ls, Nil)
  }

  // Functional (barely not "builtin").
  def splitFunctional[A](n: Int, ls: List[A]): (List[A], List[A]) =
    (ls.take(n), ls.drop(n))
}