1. Problem details:
Given a string containing only numbers 2-9 , return all letter combinations it can represent. Answers can be returned in any order . The mapping of numbers to letters is given below (same as phone keys). Note that 1 does not correspond to any letter.
Example 1:
输入：digits = "23" 输出：["ad","ae","af","bd","be","bf","cd","ce","cf"]
Example 2:
输入：digits = "" 输出：[]
Example 3:
输入：digits = "2" 输出：["a","b","c"]
hint:
0 <= digits.length <= 4