Keypad Letter Combination Variations

Given a keypad input like those on a mobile phone, enter a string of digits containing 2-9 and output all possible letter combinations that the digits could represent. For example, given input "23", return a set containing {"ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"}. Requirements: The input consists of the digits 2 to 9 and has a maximum length of 4. Example: Input: "23" Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"] Example Input 23