public static List<string> get_words_from_phone_number(string phone_number) {
        // Write your code here.
        var phoneRef = new List<List<char>>(){
            new List<char>(){' '},
            new List<char>(){'a','b','c'},
            new List<char>(){'d','e','f'},
            new List<char>(){'g','h','i'},
            new List<char>(){'j','k','l'},
            new List<char>(){'m','n','o'},
            new List<char>(){'p','q','r','s'},
            new List<char>(){'t','u','v'},
            new List<char>(){'w','x','y','z'}
        };
        var results = new List<string>();
        var slate = new List<char>();
        Helper(phone_number.Length,phoneRef,0,slate,results,phone_number);

        // result can be empty for strings having only 0 and 1
        if(results.Count == 0){
            results.Add("-1");
        }
        return results;
    }

    public static void Helper(int k, List<List<char>> phoneRef, int currentVal, List<char> slate, List<string> results, string phone_number){
        if(k == 0){
            // Don't append slate to result if slate is empty
            if(slate.Count != 0){
                results.Add(new string(slate.ToArray()).Trim());
            }
            return;
        }
        if(k < 0){
            return;
        }

        // if the number at currentVal is 0 or 1, then don't add anything to slate and move ahead
        if(phone_number[currentVal] == '0' || phone_number[currentVal] == '1'){
            Helper(k-1, phoneRef, currentVal+1, slate, results, phone_number);
            return;
        }

        int i = phone_number[currentVal] - '1';
        for(int j = 0; j < phoneRef[i].Count;j++){
            slate.Add(phoneRef[i][j]);
            Helper(k-1, phoneRef, currentVal+1, slate, results, phone_number);
            slate.RemoveAt(slate.Count-1);
        }
    }