STRINGRA (AUG17) [C++]

- [x] Issue Title is in form `Problem Code (Contest Code) [Language]`
- [x] I ensure I'm Eligible as per [ECR](https://github.com/iiitv/ChefLib/blob/master/README.md#ecr).
- [x] I got AC for this problem when the contest was live. (Not necessary, but appreciable).
- [x] I have checked that my solution is passing on Problem (in case you make some changes).
- [x] I will send my PR only after I'm assigned to this issue by any maintainer.
- [x] I ensure that I'm not assigned to more than 2 issues.
- [x] Assign this issue to me (I want to work on this).

Codechef id for ECR eligibility: https://www.codechef.com/users/udit3333

##### It's not necessary for all for points to be checked, we will look at what is required once you open an issue.

Start with the repository README's ECR section and inspect how existing CodeChef contest solutions are organized. Locate the entry point for the STRINGRA solution, add the C++ submission in the established layout, and verify that it passes the STRINGRA problem before considering the issue done.

Written by the indexing model from the issue text.