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);
		}
	}
}