电话号码的字母组合

问题描述

给定一个仅包含数字 2-9 的字符串，返回所有它能表示的字母组合（按照电话按键的字母映射）。答案可以按任意顺序返回。

数字到字母的映射

2: "abc" 3: "def" 4: "ghi" 5: "jkl" 6: "mno" 7: "pqrs" 8: "tuv" 9: "wxyz"

示例

示例1

输入：digits = "23"
输出：["ad","ae","af","bd","be","bf","cd","ce","cf"]

示例2

输入：digits = ""
输出：[]

示例3

输入：digits = "2"
输出：["a","b","c"]

约束条件

• 0 <= digits.length <= 4
• digits[i] 是范围 ['2', '9'] 的一个数字