# 17. 电话号码的字母组合
[url](https://leetcode-cn.com/problems/letter-combinations-of-a-phone-number/)
## 题目
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。答案可以按 任意顺序 返回。
给出数字到字母的映射如下（与电话按键相同）。注意 1 不对应任何字母。
```
输入：digits = "23"
输出：["ad","ae","af","bd","be","bf","cd","ce","cf"]
输入：digits = ""
输出：[]
输入：digits = "2"
输出：["a","b","c"]
```
## 方法
## code
### js
```js
let letterCombinations = digits => {
let dfs = (digits, sb) => {
if (sb.length === digits.length) {
ret
