## Description #

Difficulty: **中等**

Related Topics: Hash Table, String, Backtracking

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.

**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']`

.

## Solution #

Language: **Go**

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