PPaste!

package problems

import scala.annotation.tailrec

/**
  * Created by t.papillon on 22/12/2016.
  */
object Solutions {
  @tailrec
  def last[A](l: List[A]): Option[A] = l match {
    case List() => None
    case List(x) => Some(x)
    case _ :: xs => last(xs)
  }

  @tailrec
  def penultimate[A](l: List[A]): Option[A] = l match {
    case List() | List(_) => None
    case List(x, _) => Some(x)
    case _ :: xs => penultimate(xs)
  }

  @tailrec
  def nth[A](n: Int, l: List[A]): Option[A] = l match {
    case _ if n < 0 => None
    case List() if n >= 0 => None
    case x :: xs if n == 0 => Some(x)
    case _ :: xs => nth(n - 1, xs)
  }

  def length[A](l: List[A]): Int = {
    @tailrec
    def aux(acc: Int, l: List[A]): Int = l match {
      case List() => acc
      case _ :: xs => aux(acc + 1, xs)
    }
    aux(0, l)
  }

  def reverse[A](l: List[A]): List[A] = {
    @tailrec
    def aux(acc: List[A], l: List[A]): List[A] = l match {
      case List() => acc
      case x :: xs => aux(x :: acc, xs)
    }

    aux(List(), l)
  }
}