My SkyLine solution

sky_line.rb

class SkySkraper
  def self.area(coords)
    area_total = 0
    height_at = {}

    coords.each do |set|
      (x1, x2, height) = set

      range_exclude_last(x1, x2).each do |x|
        height_at[x] ||= 0

        if height > height_at[x]
          area_total += height - height_at[x]
          height_at[x] = height
        end
      end
    end

    area_total
  end

  def self.range_exclude_last(x1, x2)
    (x1..x2 - 1).to_a
  end
end