public static void main(String[] args) {

System.out.println(new Problem17().letterCombinations("9"));

}

public List<String> letterCombinations(String digits) {

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

if (digits.length() ==0) return res;

char c = digits.charAt(0);

char[] arr = null;

char pos = (char)('a' + 3 * (c-'2'));

if(c == '7' ) {

arr = new char[]{(char)pos, (char)(pos+1), (char)(pos+2), (char)(pos+3)};

}else

if(c == '8') {

arr = new char[]{(char)(pos+1), (char)(pos+2), (char)(pos+3)};

} else

if(c == '9') {

arr = new char[]{(char)(pos+1), (char)(pos+2), (char)(pos+3),(char)(pos+4)};

} else arr = new char[]{(char)pos, (char)(pos+1), (char)(pos+2)};

List<String> resSub = letterCombinations(digits.substring(1));

for (int i = 0; i < arr.length; i++) {

if(resSub.size() > 0) {

for (int j = 0; j < resSub.size(); j++) {

res.add(String.valueOf(arr[i]) + resSub.get(j));

}

} else {

res.add(String.valueOf(arr[i]));

}

}

return res;

}

}