[LeetCode] 42. Trapping Rain Water_hard tag: Two Pointers _leetcode

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Given n non-negative integers representing an elevation map where the width of each bar is 1, compute how much water it is able to trap after raining.

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The above elevation map 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.
【[LeetCode] 42. Trapping Rain Water_hard tag: Two Pointers】Example:

Input: [0,1,0,2,1,0,1,3,2,1,2,1] Output: 6

这个题目思路类似于木桶原理, 中间能收集到的水, 取决于左右两边较短的高度, 然后分别往中间走, 直到两者相遇. 所以设置l, r, lh(left height), rh(right height), 判断l,r指向的building
那个短就移动哪个, 比如l 指的更短, 那么l+= 1, 然后如果新building比相应的left height要短, ans += left height - building[l], 否则 height = building[l], 如果r指的更短, 那么
情况类似.

1. Constraints
1) empty, edge case, return 0
2) element will be > = 0 integer

2. Ideas

Two points:T:O(n)S: O(1)

3. Code

class Solution: def trapRainWater(self, heights): if not heights: return 0 l, r, ans = 0, len(heights) -1, 0 lh, rh = heights[l], heights[r] while l < r: if heights[l] < = heights[r]: l += 1 if heights[l] < lh: ans += lh - heights[l] else: lh = heights[l] else: r -= 1 if heights[r] < rh: ans += rh - heights[r] else: rh = heights[r] return ans

4. Test cases
1) empty
2) 1, 2, 3
3)