blue-print-solution.md

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

object P02 {
  // Again, with builtins this is easy.
  def penultimateBuiltin[A](ls: List[A]): A =
    if (ls.isEmpty) throw new NoSuchElementException
    else ls.init.last

  // But pattern matching also makes it easy.
  def penultimateRecursive[A](ls: List[A]): A = ls match {
    case h :: _ :: Nil => h
    case _ :: tail     => penultimateRecursive(tail)
    case _             => throw new NoSuchElementException
  }
  
  
  // Just for fun, let's look at making a generic lastNth function.

  // An obvious modification of the builtin solution works.
  def lastNthBuiltin[A](n: Int, ls: List[A]): A = {
    if (n <= 0) throw new IllegalArgumentException
    if (ls.length < n) throw new NoSuchElementException
    ls.takeRight(n).head
  }

  // Here's one approach to a non-builtin solution.
  def lastNthRecursive[A](n: Int, ls: List[A]): A = {
    def lastNthR(count: Int, resultList: List[A], curList: List[A]): A =
      curList match {
        case Nil if count > 0 => throw new NoSuchElementException
        case Nil              => resultList.head
        case _ :: tail        =>
          lastNthR(count - 1,
                   if (count > 0) resultList else resultList.tail,
                   tail)
      }
    if (n <= 0) throw new IllegalArgumentException
    else lastNthR(n, ls, ls)
  }
}