package q42

func trap(height []int) int {
	water := 0
	mid := 0

	// From left
	maxHeight, stash := 0, 0
	for i, h := range height {
		if h >= maxHeight {
			maxHeight = h
			water += stash
			stash = 0
			mid = i
		} else {
			stash += maxHeight - h
		}
	}

	// From right
	maxHeight, stash = 0, 0
	for i := len(height) - 1; i >= mid; i-- {
		h := height[i]
		if h >= maxHeight {
			maxHeight = h
			water += stash
			stash = 0
		} else {
			stash += maxHeight - h
		}
	}

	return water
}

var _ = trap