P02 (*) Find the last but one element of a list.

P02.scala

package com.learner.s99

/**
 * (*) Find the last but one element of a list.
 * Example:
 * scala> penultimate(List(1, 1, 2, 3, 5, 8))
 * res0: Int = 5
 */
object P02 {
  def penultimate(input: List[Any]): Any = penultimate(input, None, None)

  private def penultimate(input: List[Any], secondLast: Any, last: Any): Any = input match {
    case List() => secondLast
    case head :: tail => penultimate(tail, last, head)
  }
}

P02Spec.scala

package com.learner.s99

import com.learner.s99.P02.penultimate
import org.scalatest.{FlatSpec, MustMatchers}

class P02Spec extends FlatSpec
with MustMatchers {
  behavior of "A List"
  it must "return 7 in List(1,2,3,4,5,6,7,8) as penultimate element" in {
    penultimate(List(1, 2, 3, 4, 5, 6, 7, 8)) must equal(7)
  }

  it must "return None in List(1) as penultimate element" in {
    penultimate(List(1)) must be(None)
  }

  it must "return 'beta' in List(alpha, beta, gamma) as penultimate element" in {
    penultimate(List("alpha","beta", "gamma")) must equal("beta")
  }

  it must "return 1 in List(1, two) as penultimate element" in {
    penultimate(List(1, "two")) must equal(1)
  }

  it must "return None empty List as penultimate element" in {
    penultimate(List()) must be(None)
  }
}