http://codeluli.blogspot.fr/2013/04/trapping-rain-water.html

rainwater.scala

// Naive solution (two passes)
def volume(l: List[Int]) = {
  def peakRight(l: List[Int]) = l.foldRight(List(0)) { case (i, acc) => (i max acc.head) :: acc }.tail
  def peakLeft(l: List[Int]) = peakRight(l.reverse).reverse
  val waterLevels = peakLeft(l) zip peakRight(l) map { case (l, r) => l min r }
  val volumes = l zip waterLevels map { case (g, w) => (w - g) max 0 }
  volumes reduceLeft (_ + _)
}