S02

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

P02 (*) Find the last but one element of a list.
Example:
scala> penultimate(List(1, 1, 2, 3, 5, 8))
res0: Int = 5

def penultimate[T](xs: List[T]): T = xs match  {
  case x :: _ :: Nil => x
  case _ :: tail => penultimate(tail)
  case _ => throw new Exception("no such element")
}

def lastNthBuiltln[A](n: Int, ls: List[A]) = {
  if(n <= 0) throw new IllegalArgumentException
  if(ls.size < n) throw new NoSuchElementException
  ls.takeRight(n).head
}

def lastNth[A](n: Int, ls: List[A]): A = {
  if(n <= 0) throw new IllegalArgumentException
  if(ls.size < n) throw new NoSuchElementException
  ls match {
    case xs if(ls.length == n) => xs.head
    case _ :: tail => lastNth(n, tail)
    case Nil => throw new NoSuchElementException
  }
}