A ruby solution for the problem http://qandwhat.apps.runkite.com/i-failed-a-twitter-interview/ using tests from https://gist.github.com/igor47/7228586

water.rb

def water(x)
  water_acc(x, 0, 0, 0)
end

def water_acc(a, left, right, total)
  if a.empty?
    total
  else 
    if left <= right
      x, *xs = a
      if x < left
        water_acc(xs, left, right, total + left - x)
      else
        water_acc(xs, x, right, total)
      end
    else
      *ys, y = a
      if y < right
        water_acc(ys, left, right, total + right - y)
      else
        water_acc(ys, left, y, total)
      end
    end
  end
end

if __FILE__ == $0
  tests = [
    [[1,0,1], 1],
    [[2,0,1], 1],
    [[5,0,5], 5],
    [[0,1,0,1,0], 1],
    [[1,0,1,0], 1],
    [[1,0,1,2,0,2], 3],
    [[2,5,1,2,3,4,7,7,6], 10],
    [[5,1,0,1,], 1],
    [[2,5,1,2,3,4,7,7,6,3,5], 12],
    [[2,5,1,3,1,2,1,7,5,6], 18]
  ]

  tests.each do |x, v|
    if (water(x) == v)
      puts "TRUE: #{x}, #{v}"
    else
      puts "MISMATCH: #{x}, expected #{v} got #{water(x)}"
    end
  end
end