> ## Documentation Index
> Fetch the complete documentation index at: https://leetcode-py.wisl.dev/llms.txt
> Use this file to discover all available pages before exploring further.

> ## Agent Instructions
> leetcode-py is a Python LeetCode practice environment generator with one CLI: lcpy. It is not a service or platform.
> Each problem is a directory under leetcode/ with README.md, solution.py, test_solution.py, helpers.py, and playground.ipynb. lcpy gen creates them from JSON templates bundled with the package.
> Examples are backed by tests; copy them verbatim.

# Letter Combinations of a Phone Number

> Tested Python solution for LeetCode 17 with 12 pytest cases. Generate a practice environment with lcpy.

LeetCode 17, Medium. Topics: Hash Table, String, Backtracking. [View on LeetCode](https://leetcode.com/problems/letter-combinations-of-a-phone-number/description/).

Generate this problem as a practice environment: tested reference solution, 12 [parametrized pytest cases](/practice/testing), and a playground notebook:

```bash theme={"theme":{"light":"github-light","dark":"github-dark"}}
lcpy gen -n 17   # by problem number
lcpy gen -s letter_combinations_of_a_phone_number   # by problem name
```

## Problem

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.

### Examples

![Phone Keypad](https://assets.leetcode.com/uploads/2022/03/15/1200px-telephone-keypad2svg.png)

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

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

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

### Constraints

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

## Solution

Reference implementation from [solution.py on GitHub](https://github.com/wislertt/leetcode-py/blob/main/leetcode/letter_combinations_of_a_phone_number/solution.py), full suite in [test\_solution.py](https://github.com/wislertt/leetcode-py/blob/main/leetcode/letter_combinations_of_a_phone_number/test_solution.py):

```python theme={"theme":{"light":"github-light","dark":"github-dark"}}
class Solution:
    # Time: O(4^n)
    # Space: O(4^n)
    def letter_combinations(self, digits: str) -> list[str]:
        if not digits:
            return []

        phone = {
            "2": "abc",
            "3": "def",
            "4": "ghi",
            "5": "jkl",
            "6": "mno",
            "7": "pqrs",
            "8": "tuv",
            "9": "wxyz",
        }

        result = []

        def backtrack(i: int, path: str) -> None:
            if i == len(digits):
                result.append(path)
                return

            for letter in phone[digits[i]]:
                backtrack(i + 1, path + letter)

        backtrack(0, "")
        return result
```

## Complexity

| Time   | Space  |
| ------ | ------ |
| O(4^n) | O(4^n) |

## Tags

[Grind 75](/catalog/grind-75), [Grind](/catalog/grind), [NeetCode 150](/catalog/neetcode-150), [NeetCode 250](/catalog/neetcode-250), [NeetCode All](/catalog/neetcode).