leetcode 42 trapping rain water

leetcode_42_trapping_rain_water.rb

=begin
42. Trapping Rain Water
Given n non-negative integers representing an elevation map where the width of each bar is 1, compute how much water it can trap after raining.

Example 1:
Input: height = [0,1,0,2,1,0,1,3,2,1,2,1]
Output: 6
Explanation: The above elevation map (black section) is represented by array [0,1,0,2,1,0,1,3,2,1,2,1]. In this case, 6 units of rain water (blue section) are being trapped.

Example 2:
Input: height = [4,2,0,3,2,5]
Output: 9
=end


def trap(height)
    max = height[0]
    max_i = 0
    water = 0
    # from forward
    (1..(height.length - 1)).each do |i|
        if height[i] >= max
            ((max_i + 1)...i).each do |j|
                water += max - height[j]
            end
            max = height[i]
            max_i = i
        end
    end
    until_max_i = max_i
    # from backward
    max = height[-1]
    max_i = height.length - 1

    (until_max_i..(height.length - 2)).reverse_each do |i|
        if height[i] >= max
            ((i+1)..(max_i - 1)).each do |j|
                water += max - height[j]
            end
            max = height[i]
            max_i = i
        end
    end
    water
end

Memory beat 96%, run time only 8 ms