17. 电话号码的字母组合

   class Solution {
        String[] letter_map = {" ", "*", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};

        public List<String> letterCombinations(String digits) {
            List<String> res = new LinkedList<>();
            if (digits.length() < 1) {
                return res;
            }

            char[] chars = digits.toCharArray();
            getAllLetter("", chars, res);
            return res;
        }

        private void getAllLetter(String currStr, char[] chars, List<String> res) {
            if (currStr.length() == chars.length) {
                res.add(currStr);
                return;
            }

            int currNum = chars[currStr.length()] - '0';

            String charStr = letter_map[currNum];

            for (char c : charStr.toCharArray()) {
                getAllLetter(currStr+String.valueOf(c),chars,res);
            }
        }
    }