public List<String> letterCombinations(String digits) {
        String[] s = {"abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
        List<String> list = new ArrayList<String>();


        for (int i=0; i<digits.length(); i++){
            String tempStr = s[digits.charAt(i)-'2'];
            if (i == 0){
                for (int f=0; f<tempStr.length(); f++){
                    list.add(String.valueOf(tempStr.charAt(f)));
                }
                continue;
            }

            List<String> tempList = new ArrayList<String>();
            for (int j=0; j<list.size(); j++){
                for (int k=0; k<tempStr.length(); k++){
                    tempList.add(list.get(j)+tempStr.charAt(k));
                }
            }
            list = tempList;
        }

        return list;        
    }

    public List<String> letterCombinations(String digits) {
        // 建立映射表
        String[] table = {" ", " ", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
        StringBuilder tmp = new StringBuilder();
        List<String> res = new ArrayList<String>();
        if (digits.equals("")){
            return res;
        }
        dfs(table,0, tmp, res, digits);
        return res;
    }

    public static void dfs(String[] table, int dep, StringBuilder tmp,List<String> list, String digits){
        if (dep == digits.length()){
            list.add(tmp.toString());//算出了结果
        }else{
            String tempStr = table[digits.charAt(dep)- '0'];
            for (int i=0; i<tempStr.length(); i++){
                tmp.append(tempStr.charAt(i));
                dfs(table, dep+1, tmp, list, digits);
                tmp.deleteCharAt(tmp.length()-1);
            }
        }
    }