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.
範例 1:
輸入： digits = "23" 輸出： ["ad","ae","af","bd","be","bf","cd","ce","cf"]
範例 2:
輸入： digits = "" 輸出： []
限制條件：
0 <= digits.length <= 4digits[i] is a digit from 2 to 9.