Create 1512. good pairs.cpp

[Sidd7000]

Solved issue #439

Pull Request Template

Description

Please include a summary of the problem and about the approach to solve the solution. Please also include relevant motivation and context.
List any dependencies that are required for this change.

Put check marks:

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:

• [x ] 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!