12. Letter Combinations of a Phone Number
Given a string containing digits from 2 to 9, return all possible letter combinations that the number could represent.
Example 1:
Input: digits = "23" Output: ["ad","ae","af","bd","be","bf","cd","ce","cf"]
Example 2:
Input: digits = "" Output: []
Constraints:
0 <= digits.length <= 4digits[i] is a digit from 2 to 9.