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. 2 -> abc 3 -> def 4 -> ghi 5 -> jkl 6 -> mno 7 -> pqrs 8 -> tuv 9 -> wxyz