Added C++ solution for 0287 - Find The Duplicate.

Change Summary

The purpose of this PR is to add coding solution in C++ for the LeetCode Find The Duplicate problem.

If you haven't fulfilled the below requirements or even delete the entire checklist, your PR won't be reviewed and will be closed without notice.

[x] This Pull Request is all my own work. (You'll be blacklisted if you are caught for plagiarism.)
[x] I've read CONTRIBUTING.md
[x] I've applied LaTex for all variables, formulas and time / space complexity instead of using backticks
[x] I've started the app locally and verified all the content and all links (if applicable) are accessible correctly
[x] I've included Complexity Analysis (Time Complexity & Space Complexity).
[x] I've written my explanation well and it is easy to understand for beginners

[x] I've read and followed the Solution Template
[x] I've formatted my code well with K&R Coding style
[x] I've confirmed that comments are put above each line rather than writing on the same line.
[x] I've included a meaningful approach name for my solution. e.g. ## Approach 1: Two Pointers.