Added trapping rainwater in cpp

[nikkilaprakash]

Pull Request Template

Description

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.
Input: height = [0,1,0,2,1,0,1,3,2,1,2,1]
Output: 6

For the solution, we calculate the maximum heights to the left and the right, and decide the amount of stagnant water accordingly

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Have you made changes in README file ?

• Added problem & solution under correct topic.
• Specified Space & Time complexity.
• Specified difficulty level, tag & Note(if any).

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Please also note any relevant details for your test configuration.

• Test A
• Test B

Make sure all below guidelines are followed else PR will get Reject:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code so that it is easy to understand
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules

[welcome]

I can tell this is your first pull request! Thank you I'm so honored. 🎉🎉🎉 I'll take a look at it ASAP!