this is think is O(n2)

water-container.rb

# @param {Integer[]} height
# @return {Integer}
def max_area(heights)
  max = 0
  heights.each_with_index do |y, x|
    heights.each_with_index do |y1, x1|
      width = (x - x1).abs
      area = [y, y1].min * width
      max = area if area >= max
    end
  end
  max
end

example = [1,8,6,2,5,4,8,3,7]
puts max_area(example)