Letter Combinations of a Phone Number

The 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 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’] Explanation From the description of the problem, we learn that we are given a string with digits from 2-9 and we need to return all possible combinations that the number could represent. ...