Ruby solution for http://qandwhat.apps.runkite.com/i-failed-a-twitter-interview/. A bit more verbose but perhaps slightly more efficient solution than others? Nope, just more verbose.

i_failed_a_twitter_interview.rb

walls = [2,5,1,2,3,4,7,7,6]

# array of max values of elements to the left for each element
def running_max(_walls)
  maxmin = 0
  _walls.inject([]) do |maxes, c|
    maxes << if (c < maxmin) then
              maxmin
            else
              maxmin = c
            end
    maxes
  end
end

running_max_left = running_max(walls)
running_max_right = running_max(walls.reverse).reverse

water_volume = 0
walls.each_with_index do |f, i|
  min_wall = [running_max_left[i], running_max_right[i]].min
  water_volume += [min_wall - f, 0].max
end

puts "Total water volume is #{water_volume}"