## Letter Combinations of a Phone Number

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 digit to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters.

<p align="left">
  <img src="../../assets/Telephone-keypad2.png" alt="telefone keypad">
</p>

### Examples

**Example 1:**

```text
Input: digits = "23"
Output: ["ad","ae","af","bd","be","bf","cd","ce","cf"]
```

**Example 2:**

```text
Input: digits = ""
Output: []
```

**Example 3:**

```text
Input: digits = "2"
Output: ["a","b","c"]
```

### Constraints

- `0 <= digits.length <= 4`
- `digits[i] is a digit in the range ['2', '9'].`

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### Notes

- N/A