42-Trapping-Rain-Water

1. Trapping Rain Water

• 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:
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 3 |           __ _________||||__ __ __
 2 |    __ ___||||__     __|||||||__||||__
 1 |   ||||   |||||||   |||||||||||||||||||
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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
0 <= n <= 3 * 10^4
0 <= height[i] <= 10^5