*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"].

public class T17 {

    public List<String> letterCombinations(String digits) {
        HashMap<Character, String> hashMap = new HashMap<>();
        hashMap.put('2', "abc");
        hashMap.put('3', "def");
        hashMap.put('4', "ghi");
        hashMap.put('5', "jkl");
        hashMap.put('6', "mno");
        hashMap.put('7', "pqrs");
        hashMap.put('8', "tuv");
        hashMap.put('9', "wxyz");
        List<String> list = new ArrayList<>();
        if (digits == null || digits.length() == 0) {
            return list;
        }
        getLeffters(digits, list,new StringBuilder(), 0,hashMap);
        return list;

    }

    private void getLeffters(String digits, List<String> list, StringBuilder stringBuilder, int i,HashMap<Character,String> hashMap) {
        if (i == digits.length()) {
            list.add(stringBuilder.toString());
            return;
        }
        String str = hashMap.get(digits.charAt(i));

        for (int j = 0; j < str.length(); j++) {
            stringBuilder.append(str.charAt(j));
            getLeffters(digits, list, stringBuilder, i + 1, hashMap);
            stringBuilder.deleteCharAt(stringBuilder.length() - 1);
        }
    }
}