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.

使用递归求解。

    char[][] letters = {{'a','b','c'},{'d','e','f'},{'g','h','i'},{'j','k','l'},{'m','n','o'},{'p','q','r','s'},{'t','u','v'},{'w','x','y','z'}};
    public void getCombi(String digits, int cur, int len, String str, List<String> list) {
        if (cur == len) {
            list.add(str);
        } else {
            int index = digits.charAt(cur) - '2';
            int leng  = letters[index].length;
            for (int i = 0; i < leng; i++) {
                String newStr = str +  letters[index][i];
                getCombi(digits, cur + 1, len, newStr, list);
            }
            
        }
    }
    public List<String> letterCombinations(String digits) {
        List<String> list = new ArrayList<String>();
        String str = "";
        int len = digits.length();
        getCombi(digits, 0, len, str, list);
        return list;
    }