Given a string containing digits from 2 - 9 inclusive, write a program to return all possible letter combinations that the number could represent.

Problem Note

  • Your answer could be in any order you want.
  • A mapping of digit to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters.

Example

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