S07

P06はしょうもないので省略。

S-99: Ninety-Nine Scala Problems
を試してみる。

// P07 (**) Flatten a nested list structure.
// Example:
// scala> flatten(List(List(1, 1), 2, List(3, List(5, 8))))
// res0: List[Any] = List(1, 1, 2, 3, 5, 8)

def flatten(xs: List[Any]): List[Any] = {
  def recursive(rest: List[Any], result: List[Any]): List[Any] = rest match {
    case Nil => result
    case head :: tail => 
      head match {
        case Nil => recursive(tail, result)
        case h :: t => recursive(tail, recursive(t, recursive(List(h), result)))
        case notaList => recursive(tail, result ::: List(notaList))
      } 
    case notaList => result ::: List(notaList)
  }
  recursive(xs, Nil)
}

解答例
ちくしょー！反則だ（笑