17. 电话号码的字母组合

给定一个仅包含数字 2-9 的字符串，返回所有它能表示的字母组合。 给出数字到字母的映射如下（与电话按键相同）。注意 1 不对应任何字母。

示例:

输入：“23”输出：[“ad”, “ae”, “af”, “bd”, “be”, “bf”, “cd”, “ce”, “cf”].

说明:

尽管上面的答案是按字典序排列的，但是你可以任意选择答案输出的顺序。

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
public class LetterCombinations {
    public List<String> letterCombinations(String digits) {
        List<String> result = new ArrayList<>();
        if (digits == null || digits.length() == 0) {
            return result;
        }
        Map<Character, Character[]> map = new HashMap<>();
        map.put('2', new Character[]{'a','b','c'});
        map.put('3', new Character[]{'d','e','f'});
        map.put('4', new Character[]{'g','h','i'});
        map.put('5', new Character[]{'j','k','l'});
        map.put('6', new Character[]{'m','n','o'});
        map.put('7', new Character[]{'p','q','r', 's'});
        map.put('8', new Character[]{'t','u','v'});
        map.put('9', new Character[]{'w','x','y', 'z'});
        select(result, 0, new StringBuffer(), map, digits);
        return result;
    }
    private void select(List<String> result, int index, StringBuffer word, Map<Character, Character[]> map, String digits) {
        if (word.length() == digits.length()) {
            result.add(word.toString());
        } else {
            char c = digits.charAt(index);
            Character[] characters = map.get(c);
            for (Character character : characters) {
                word.append(character);
                select(result, index + 1, word, map, digits);
                word.deleteCharAt(index);
            }
        }
    }
}