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.
这是回溯问题，下面是JavaScript代码编写的递归版和迭代版的代码
17. Letter Combinations of a Phone Number
原文：http://www.cnblogs.com/bubbleStar/p/6065101.html