Phone permutations interview question

interview_question_phone_permutations.cs

using System;
using System.Collections.Generic;

// Image of a telephone's keypad: https://upload.wikimedia.org/wikipedia/commons/thumb/7/73/Telephone-keypad2.svg/2880px-Telephone-keypad2.svg.png
// Problem: Given phone number, output all possible letter values or "words".
// Example:
// Input: 123-000
// Output: 1ad-000 1ae-000 1af-000 1bd-000 1be-000 1bf-000 1cd-000 1ce-000 1cf-000
//
// Example:
// Input: 523
// 5 - jkl
// 2 - abc
// 3 - def
// Output: 
// jad, jae, jaf, jbd, jbe, jbf, jcd, jce, jcf, 
// jad, jae, jaf, jbd, jbe, jbf, jcd, jce, jcf, j -> k
// 1: 1
// 2: abc
// 3: def
// 4: ghi
// 5: jkl
// 6: mno
// 7: pqrs
// 8: tuv
// 9: wxyz
// 0: 0
class Solution
{
    static string[] symbols = new[] { "0", "1", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz", };
    static void Main(string[] args)
    {
        // Solution1();
        Solution2();
    }

    static void Solution2()
    {
        //const string input = "1";
        //const string input = "234";
        const string input = "23";
        // output: ad, ae, af, bd, be, bf 

        Console.WriteLine($"Input: '{input}'");
        List<byte> digits = new();
        foreach (var ch in input)
        {
            // filter only valid digits
            if (byte.TryParse("" + ch, out var d) && d >= 0 & d <= 9)
                digits.Add(d);
            else
                Console.WriteLine("Skipping non digit '{ch}'");
        }

        if (digits.Count == 0)
        {
            Console.WriteLine("No valid digits found");
            return;
        }

        var firstSymbols = symbols[digits[0]];
        if (digits.Count == 1)
        {
            Console.WriteLine($"A single result is {firstSymbols}");
            return;
        }

        List<string> results = new();

        Permutate(digits, results, firstSymbols);

        Console.WriteLine($"All permutation results are: [{string.Join(", ", results)}]");

        static void Permutate(List<byte> digits, List<string> results, string digitSymbols, string res = "", int i = 0)
        {
            ++i;
            if (i >= digits.Count)
            {
                foreach (var s in digitSymbols)
                    results.Add(res + s);
            }
            else
            {
                var nextDigitSymbols = symbols[digits[i]];
                foreach (var s in digitSymbols)
                    Permutate(digits, results, nextDigitSymbols, res + s, i);
            }
        }
    }

    static void Solution1()
    {
        const string input = "123";
        var n1 = int.Parse("" + input[0]);
        var n2 = int.Parse("" + input[1]);
        var n3 = int.Parse("" + input[2]);
        var s1 = symbols[n1];
        var s2 = symbols[n2];
        var s3 = symbols[n3];
        foreach (var a in s1)
        {
            foreach (var b in s2)
            {
                foreach (var c in s3)
                {
                    Console.WriteLine($"{a}{b}{c}");
                }
            }
        }

        Console.WriteLine($"{n1}, {n2}, {n3}");
        Console.WriteLine($"{s1}, {s2}, {s3}");
    }
}

The newer solution is here: https://dotnetfiddle.net/D6rO5F
For input 23 it outputs:

Input: '23'
All permutation results are: [ad, ae, af, bd, be, bf, cd, ce, cf]