Java [leetcode 17]Letter Combinations of a Phone Number

题目描述：

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.

解题思路：

递归法解题之。从第一个字符串开始确认结果，然后递归地确认余下各项结果。

参考博客：http://blog.csdn.net/china_wanglong/article/details/38495355

代码如下：

public class Solution {

    public List<String> letterCombinations(String digits) {

		List<String> result = new ArrayList<String>();

		String[] map = new String[] { "", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz" };

		char[] tmp = new char[digits.length()];

		if(digits.length() == 0)

			return result;

		rec(digits, 0, tmp, map, result);

		return result;

	}

	public void rec(String digits, int index, char[] tmp, String[] map, List<String> result){

		if(index == digits.length()){

			result.add(new String(tmp));

			return;

		}

		char tmpChar = digits.charAt(index);

		for(int i = 0; i < map[tmpChar - '0'].length(); i++){

			tmp[index] = map[tmpChar - '0'].charAt(i);

			rec(digits, index + 1, tmp, map, result);

		}

	}

}