S20

S-99: Ninety-Nine Scala Problemsを試してみる。
勉強のためにListのAPIはなるべく使わない方向。

// P20 (*) Remove the Kth element from a list.
// Return the list and the removed element in a Tuple. Elements are numbered from 0.
// Example:
// scala> removeAt(1, List('a, 'b, 'c, 'd))
// res0: (List[Symbol], Symbol) = (List('a, 'c, 'd),'b)

def removeAt[T](i: Int, xs: List[T]) =
  if(i < 0 || xs.size <= i) throw new NoSuchElementException
  else (xs.zipWithIndex.filter{_._2 != i}.map{_._1}, xs(i))

def removeAtR[T](i: Int, xs: List[T]) = {
  def r(i: Int, xs: List[T], acc: List[T]): (List[T], T) = (i, xs) match {
    case (0, h :: t) => (acc.reverse ::: t, h)
    case (i, h :: t) => r(i - 1, t, h :: acc)
  }
  if(i < 0 || xs.size <= i) throw new NoSuchElementException else r(i, xs, Nil)
}

解答例の上のやつ、いいね。