Answer Happy

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 digit to letters (just like on the telephone
buttons) is given below. Note that 1 does not map to any
letters.
Example 1:
Example 2:
Example 3:
Constraints:
class Solution:
def letterCombinations(self, digits: str) ->
List[str]:
Implementation in Python3