package trapping_rain_water

func Trap(heights []int) int {
	// Calculate area taken up by the boundary.
	boundary_area := 0
	for _, height := range heights {
		boundary_area += height
	}

	// Calculate the total area taken up by the boundary and the water.
	total_area := 0
	left := 0
	left_height := 0
	right := len(heights) - 1
	right_height := 0

	for left <= right {
		left_height = max(left_height, heights[left])
		right_height = max(right_height, heights[right])

		if left_height < right_height {
			total_area += left_height
			left++
		} else {
			total_area += right_height
			right--
		}
	}

	return total_area - boundary_area
}