Create Watermelon.cpp

[SamyakVerma]

Please place an x inside the [brackets] in the checklist below

Make sure all the item in the checklist have been done before making the PR

• [x] You have starred the repo
• [x] If you have added a new folder, there are no spaces in the name of the folder
• [x] The link to the problem set is present on the first line of the code file
• [x] Your code (solution) in your code file(s) is accepted by the problemset on CodeForces