P05 (*) Reverse a list.

P05.scala

package com.learner.s99

/**
 * P05 (*) Reverse a list.
 * Example:
 * scala> reverse(List(1, 1, 2, 3, 5, 8))
 * res0: List[Int] = List(8, 5, 3, 2, 1, 1)
 */
object P05 {
  def reverse[T](l:List[T]): Option[List[T]] = reverse(l, Nil)

  private def reverse[T](l:List[T], reversed:List[T]): Option[List[T]] = {
    l match {
      case Nil => if (reversed.nonEmpty) Some(reversed) else None
      case head::tail => reverse(tail, head::reversed)
    }
  }
}

P05Spec.scala

package com.learner.s99

import com.learner.s99.P05.reverse
import org.scalatest.{MustMatchers, FlatSpec}

class P05Spec extends FlatSpec
with MustMatchers {
  behavior of "A List"

  it must "return Some(List(3,2,1)) for List(1,2,3) as reversed List" in {
    reverse(List(1, 2, 3)) must be(Some(List(3, 2, 1)))
  }

  it must "return Some(List(1)) for List(1,2,3) as reversed List" in {
    reverse(List(1)) must be(Some(List(1)))
  }

  it must "return None for empty list as reversed List" in {
    reverse(List()) mustBe None
  }
}