Leetcode 17 - phone number combination - need to work on the code, remove if/ else statement, make the code more flat.

Leetcode17PhoneNumberDFS3.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace _17LetterCombinationOfAPhoneNUmber_DFS
{
    /*
     * Leetcode 17:
     * 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"].
     * 
     * Telephone dial pad:
     *                     0  1   2     3     4     5     6     7      8     9 
     * String[] keyboard={"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
     */
    class Solution
    {
        static void Main(string[] args)
        {
            IList<string> list = letterCombination("23");
            // test result: "abc", "def", so 3x3 = 9 cases. 
        }

        public static IList<string> letterCombination(string digits)
        {
            IList<string> list = new List<string>();
            if (digits == null || digits.Length == 0)
                return list;

            string[] keyboard = { "", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz" };

            StringBuilder sb = new StringBuilder(); 
            buildResult(list, keyboard, digits, sb, 0);  // think about design here

            return list;
        }

        /*
         * Julia's practice on April 26, 2016
         * Need to rewrite to get more experience. 
         * 2nd practice: 
         *   Lessons learned:
         *   1. Use another tmpS variable to save StringBuilder variable for next iteration
         *   
         * Static analysis notes: 
         * 1. line 70, 71, use explanation variables c, s1, 
         * 2. miss the bug - 
         *    line 68: index < len, not "len - 1", 
         *    Julia, you have to allow the string complete, then, add it to the list. 
         * 3. add simple test case next to the function argument, help to complete the coding. 
         * 
         * one more note:
         * Add basic case - when the string is ready to output and then add to the list. 
         * This way, the code is more easy to read and understand. 
         * 
         */
        private static void buildResult(
            IList<string>   list, 
            string[]        keyboard, 
            string          digits, // test case: "23"
            StringBuilder   yourOutput, 
            int             index)  // test case: "23", index value 0 and 1
        {
            // need to do the backtracking, need to do the recursive solution            
            if (index == digits.Length)
            {
                // add to the list
                list.Add(yourOutput.ToString());
                return; 
            }
           
            char    c = digits[index];
            string s1 = keyboard[c - '0'];

            foreach (char nextChar in s1)
            {
                string tmpS = yourOutput.ToString(); 

                yourOutput.Append(nextChar);                    
                buildResult(list, keyboard, digits, yourOutput, index + 1);

                yourOutput = new StringBuilder(tmpS);                     
            }                                
        }        
    }
}