LeetCode

题目：

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.

思路：递归。

package recursion;

import java.util.ArrayList;
import java.util.List;

public class LetterCombinationsOfAPhoneNumber {

    public List<String> letterCombinations(String digits) {
        String[] map = { " ", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz" };
        List<String> res = new ArrayList<String>();
        if (digits == null || digits.length() == 0) return res;
        res.add("");
        return DFS(map, res, digits, 0);
    }
    
    private List<String> DFS(String[] map, List<String> subRes, String digits, int pos) {
        if (pos == digits.length()) return subRes;
        int index = digits.charAt(pos) - '0';
        List<String> res = new ArrayList<String>();
        for (String s : subRes) {
            for (int j = 0; j < map[index].length(); ++j) {
                res.add(s + map[index].charAt(j));
            }
        }
        
        return DFS(map, res, digits, ++pos);
    }
    
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        LetterCombinationsOfAPhoneNumber l = new LetterCombinationsOfAPhoneNumber();
        System.out.println(l.letterCombinations("23"));
    }

}