I want to add LCS c++ code

#### This issue is about [Longest Common Subsequence].

- [X] Issue Title is in form `LCS [C++]`.
- [X] I searched or browsed the repo’s other issues to ensure this is not a duplicate.
- [X] This Algo/DS is worth adding to this repository.
- [X] Assign this issue to me (I want to work on this).
- [X] I ensure that I am not already assigned to 2 or more issues.
- [X] I will send my PR only after I'm assigned to this issue by any maintainer.
- [X] I understand that I will be unassigned from this issue if I fail to send a PR for this within a week.

No file or test is named. Start by reviewing existing algorithm implementations and any C++ examples in the repository to identify placement and conventions; add the Longest Common Subsequence implementation there, and verify it with the repository’s applicable checks and an example covering the expected result.

Written by the indexing model from the issue text.