# Given n non-negative integers representing an
# elevation map where the width of each bar is
# 1, compute how much water it is able to trap
# after raining.

def trappableVolume(depthMap)
  maxLeft = Array.new(depthMap.size, 0)
  maxRight = Array.new(depthMap.size, 0)

  maxObserved = depthMap[0]
  depthMap.each_with_index do |x, i|
    maxLeft[i] = maxObserved = [x, maxObserved].max
  end

  maxObserved = depthMap.last
  depthMap.to_enum.with_index.reverse_each do |x, i|
    maxRight[i] = maxObserved = [x, maxObserved].max
  end

  depthMap.zip(maxLeft, maxRight).collect do |a|
    [a[1], a[2]].min - a[0]
  end.reduce(0, :+)
end