Telephone Coder

Solution.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace ConsoleApp2
{
    public static class Solution
    {
        private static readonly Dictionary<int, IEnumerable<string>> NumbersToLetters = new()
        {
            {2, new[] {"a", "b", "c"}},
            {3, new[] {"d", "e", "f"}},
            {4, new[] {"g", "h", "i"}},
            {5, new[] {"j", "k", "l"}},
            {6, new[] {"m", "n", "o"}},
            {7, new[] {"p", "q", "r", "s"}},
            {8, new[] {"t", "u", "v"}},
            {9, new[] {"w", "x", "y", "z"}}
        };

        public static IEnumerable<string> GetPasswordCombination(string input)
        {
            var letters = GetNumbersFromNumberAsString(input).Select(x => NumbersToLetters[x]);
            return letters.Aggregate((x, y) => x.CombineElements(y));
        }

        private static IEnumerable<int> GetNumbersFromNumberAsString(string input)
        {
            if (!int.TryParse(input.Reverse(), out var number))
                throw new InvalidOperationException();

            while (number > 0)
            {
                yield return number % 10;
                number /= 10;
            }
        }
    }

    public static class EnumerableExtensions
    {
        public static IEnumerable<string> CombineElements(this IEnumerable<string> source,
            IEnumerable<string> forCombine)
        {
            return from i in source from j in forCombine select i + j;
        }
    }

    public static class StringExtensions
    {
        public static string Reverse(this string input)
        {
            var arr = input.ToCharArray();
            Array.Reverse(arr);
            return new string(arr);
        }
    }
}

Tests.cs

using System.Collections.Generic;
using ConsoleApp2;
using FluentAssertions;
using NUnit.Framework;

namespace Tests
{
    public class Tests
    {
        [TestCase("2", new[] {"a", "b", "c"})]
        [TestCase("23", new[] {"ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"})]
        [TestCase("234",
            new[]
            {
                "adg", "adh", "adi", "aeg", "aeh", "aei", "afg", "afh", "afi", "bdg", "bdh", "bdi", "beg", "beh", "bei",
                "bfg", "bfh", "bfi", "cdg", "cdh", "cdi", "ceg", "ceh", "cei", "cfg", "cfh", "cfi"
            })]
        public void GetPasswordCombination_ShouldPass(string input, IEnumerable<string> expected)
        {
            var result = Solution.GetPasswordCombination(input);

            result.Should().BeEquivalentTo(expected);
        }

        [TestCase("123", "321")]
        [TestCase("abra", "arba")]
        public void Reverse_ShouldPass(string input, string expected)
        {
            var result = input.Reverse();

            result.Should().BeEquivalentTo(expected);
        }

        [TestCase(new[] {"a", "b", "c"}, new[] {"a", "b", "c"},
            new[] {"aa", "ab", "ac", "ba", "bb", "bc", "ca", "cb", "cc"})]
        public void CombineElements_ShouldPass(IEnumerable<string> first, IEnumerable<string> second,
            IEnumerable<string> expected)
        {
            var result = first.CombineElements(second);

            result.Should().BeEquivalentTo(expected);
        }
    }
}