LeetCode-in-All

42. Trapping Rain Water

Hard

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

Constraints:

• n == height.length
• 1 <= n <= 2 * 104
• 0 <= height[i] <= 105

Solution

func trap(height []int) int {
	if height == nil {
		return 0
	}
	l, r := 0, len(height)-1
	max_left, max_right := height[l], height[r]
	res := 0
	for l < r {
		if max_left < max_right {
			l++
			max_left = max(max_left, height[l])
			res += max_left - height[l]
		} else {
			r--
			max_right = max(max_right, height[r])
			res += max_right - height[r]
		}
	}
	return res
}

func max(a, b int) int {
	if a > b {
		return a
	}
	return b
}

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