[17. Letter Combinations of a Phone Number](https://leetcode.com/problems/letter-combinations-of-a-phone-number/)
### 題目描述
Given a string containing digits from `2-9` inclusive, return all possible letter combinations that the number could represent. Return the answer in **any order**.
A mapping of digits to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters.
![](https://assets.leetcode.com/uploads/2022/03/15/1200px-telephone-keypad2svg.png =50%x)
### 範例
**Example 1:**
```
Input: digits = "23"
Output: ["ad","ae","af","bd","be","bf","cd","ce","cf"]
```
**Example 2:**
```
Input: digits = ""
Output: []
```
**Example 3:**
```
Input: digits = "2"
Output: ["a","b","c"]
```
**Constraints**:
* 0 <= `digits.length` <= 4
* `digits[i]` is a digit in the range `['2', '9']`.
### 解答
### Reference
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