Added : Code for Key Pad Problem in Java

[ankit-95]

Description

Given a string containing digits from 2-9 inclusive, return all possible letter combinations that the number could represent. Return the answer in any order. A mapping of digits to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters.

Fixes: #623

Type of change

Please delete options that are not relevant

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] This change requires a documentation update

Checklist:

• [x] I have read the contribution guidelines before making this PR.
• [x] I have performed a self-review of my own code

[kaustubhgupta]

Thanks for your contribution :)

[ankit-95]

@kaustubhgupta Thanks for giving the opportunity to contribute.