[LeetCode] 17. 电话号码的字母组合 ☆☆☆(回溯)

//me
class Solution {
    static Map<String, String> map = new HashMap<>();
    
    static {
        map.put("2", "abc");
        map.put("3", "def");
        map.put("4", "ghi");
        map.put("5", "jkl");
        map.put("6", "mno");
        map.put("7", "pqrs");
        map.put("8", "tuv");
        map.put("9", "wxyz");
    }

    List<String> output = new ArrayList<String>();
    public List<String> letterCombinations(String digits) {
        if (digits == null || digits.length() <= 0) {
            return output;
        }
        help(output, "", digits, 0);
        return output;
    }
    
    public void help(List<String> list, String tempStr, String digits, int pos) {
        if (pos == digits.length() && tempStr.length() == pos) {
            list.add(tempStr);
            return;
        }
        for (int i = pos; i < digits.length(); i++) {
            String curStr = digits.charAt(i) + "";
            String mapStr = map.get(curStr);
            for (int k = 0; k < mapStr.length(); k++) {
                tempStr += mapStr.charAt(k);
                help(list, tempStr, digits, i + 1);
                tempStr = tempStr.substring(0, tempStr.length() - 1);
            }
        }
    }
}