class Solution {
    // 所有可能，全排列，回溯
    List<String> res = new ArrayList<>();
    String[] numbers = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
    public List<String> letterCombinations(String digits) {
          if (digits.length() == 0) {
            return res;
          }
          backTracking(digits, 0);
          return res;
    }
    StringBuffer sb = new StringBuffer();
    public void backTracking(String digits, int num) {
       if (num == digits.length()) {
        res.add(sb.toString());
        return;
       }
        int index = digits.charAt(num) - '0'; // 这里-'0'得到数字，不能用Inter.valueOf(),这是将字符串转为数字
        String tmp = numbers[index];
        for (int i = 0; i < tmp.length(); i++) {
            sb.append(tmp.charAt(i));
            backTracking(digits, num + 1);
            sb.deleteCharAt(sb.length() - 1);           
        }
        
    }
   
}