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.

Input:Digit string "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.

题目：给定一个数字字符串，返回所有数字能代表的字母的组合。

思路：将数字和对应的字母放到一个哈希表中。

	static final HashMap<Character, String> map = new HashMap<Character, String>() {
		{
			put('2', "abc");
			put('3', "def");
			put('4', "ghi");
			put('5', "jkl");
			put('6', "mno");
			put('7', "pqrs");
			put('8', "tuv");
			put('9', "wxyz");
		}
	};

	public static List<String> letterCombinations(String digits) {
		List<String> res = new ArrayList<String>();
		res.add("");

		for (int i = 0; i < digits.length(); i++) {
			List<String> tmp = new ArrayList<String>();
			for (String str : res) {
				String letters = map.get(digits.charAt(i));
				for (int j = 0; j < letters.length(); j++)
					tmp.add(str + letters.charAt(j));
			}
			res = tmp;
		}
		return res;
	}

LeetCode——Letter Combinations of a Phone Number