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 digits to letters (just like on telephone buttons) is given below. Note that `1` does not map to any letters. ``` 2 -> "abc" 3 -> "def" 4 -> "ghi" 5 -> "jkl" 6 -> "mno" 7 -> "pqrs" 8 -> "tuv" 9 -> "wxyz" ``` **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"] ```