remaining_water.rb

def remaining_water(height)
  # some boundary checking, height is nil, height is emtpy? 
  
  left_point = 0
  right_point = 0
  
  water_remaining = 0
  left = 0
  right = height.length - 1
  
  while left < right
    if height[left] < height[right]
      left_point = [height[left], left_point].max
      water_remaining += (left_point - height[left])
      left += 1
    else 
      right_point = [height[right], right_point].max
      water_remaining += (right_point - height[right])
      right -= 1
    end
  end
  
  water_remaining
end

puts remaining_water([3,0,0,2,0,5]) # 10
puts remaining_water([5,0,2,0,0,3]) # 10
puts remaining_water([0,3,0,0,2,0,5,0,2]) # 12
puts remaining_water([0,2,0,3,2,1,1,2,1]) # 4