[Leetcode] Letter Combinations of Phone Number

Problem

Given a digit string, return all possible letter combinations that the number could represent.

A mapping of digit to letters (just like on the telephone buttons) is given below.

[1,2,3,4,5,6,7,8,9,9] = {"",   "abc",  "def", "ghi", "jkl",  "mno", "pqrs","tuv",  "wxyz","0" };

Input:Digit string "23"

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