Given a string containing digits from 2-9 inclusive, 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. Note that 1 does not map to any letters.
Example:
Input: "23" Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].
Note:
Although the above answer is in lexicographical order, your answer could be in any order you want.
这道题可以使用回溯来解决：每次取出一个数字对应的字母列表，遍历追加到上一次的字母组合中。依次进行，直到数字取完为止。
link:{sourcedir}/_0017_LetterCombinationsOfAPhoneNumber.java[role=include]