leetcode solution for letter combinations of a phone number

[Salonii02]

This is a(n):

• [x] New algorithm
• [ ] Update to an existing algorithm
• [ ] Error
• [x] Proposal to the Repository

Details:

Problem link https://leetcode.com/problems/letter-combinations-of-a-phone-number/

[Salonii02]

I would like to work on this issue. I am participant of gssoc20. can you please assign me this @arnavb