Problem statement (online source)

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"].

Note:

Although the above answer is in lexicographical order, your answer could be in any order you want.

I have seen and read the reviews for the above problem, and made the modifications suggested. I did see the comment on using LINQ, I am learning how to use them and don't fully understand it yet. I successfully submitted my answer to LeetCode, but I would like a review to see what else I can do to improve the code. I noticed the code uses more memory than most submissions on LeetCode and I wonder if I can find out why here.

My code is based on a tutorial I watched. While it was in Java, I typed my code in C#. It recursively calls the method that looks into the objects of a Dictionary and concatenates the data inside them in the desired format. I am new to programming so I looked up stuff online and made some assumptions on equivalent data structures in C#. For example, the original code used a HashMap, and saw a recommendation online to used Dictionary<T, T> in C#.

public class Solution {
public IList<string> LetterCombinations(string digits) {
    List<String> result = new List<String>();
    if (String.IsNullOrEmpty(digits)){
        return result;
        throw new ArgumentNullException(nameof(digits));
    }
    Dictionary<Char, Char[]> lettersMap = new Dictionary<Char, char[]>();
        lettersMap.Add('1', null);
        lettersMap.Add('2', new[] { 'a', 'b', 'c' });
        lettersMap.Add('3', new[] { 'd', 'e', 'f' });
        lettersMap.Add('4', new[] { 'g', 'h', 'i' });
        lettersMap.Add('5', new[] { 'j', 'k', 'l' });
        lettersMap.Add('6', new[] { 'm', 'n', 'o' });
        lettersMap.Add('7', new[] { 'p', 'q', 'r', 's' });
        lettersMap.Add('8', new[] { 't', 'u', 'v' });
        lettersMap.Add('9', new[] { 'w', 'x', 'y', 'z' });
        lettersMap.Add('0', null);
        StringBuilder sb = new StringBuilder();
        int pos = 0;
        LetterCombinationsFunction(digits, sb, lettersMap, result, pos);
    return result;
}
private static List<String> LetterCombinationsFunction(String digits, StringBuilder sb,
                Dictionary<Char, Char[]> lettersMap, List<String> result, int pos)
    { 
        if (sb.Length == digits.Count())
        {
            result.Add(sb.ToString());
            return result;
        }
        lettersMap.TryGetValue(digits[pos], out char[] values);        
            foreach (var v in values)
            {  
                sb.Append(v);
                LetterCombinationsFunction(digits, sb, lettersMap, result, pos+1);
                sb.Remove(sb.Length - 1, 1);
            }      
        return result;
    }
}