How can I print out all possible letter combinations a given phone number can represent?

Question

I just tried for my first programming interview and one of the questions was to write a program that given a 7 digit telephone number, could print all possible combinations

Answer 1

This is the C# port of this answer.

Code

public class LetterCombinations
{
    private static readonly Dictionary Representations = new Dictionary
    {
       {"2", "abc" },
       {"3", "def" },
       {"4", "ghi" },
       {"5", "jkl" },
       {"6", "mno" },
       {"7", "pqrs" },
       {"8", "tuv" },
       {"9", "wxyz" },
    };

    public static List FromPhoneNumber(string phoneNumber)
    {
        var result = new List { string.Empty };

        // go through each number in the phone
        for (int i = 0; i < phoneNumber.Length; i++)
        {
            var pre = new List();
            foreach (var str in result)
            {
                var letters = Representations[phoneNumber[i].ToString()];
                // go through each representation of the number
                for (int j = 0; j < letters.Length; j++)
                {
                    pre.Add(str + letters[j]);
                }
            }
            result = pre;
        }

        return result;
    }
}

Unit Tests

public class UnitTest
{
    [TestMethod]
    public void One_Digit_Yields_Three_Representations()
    {
        var sut = "2";

        var expected = new List{ "a", "b", "c" };
        var actualResults = LetterCombinations.FromPhoneNumber(sut);

        CollectionAssert.AreEqual(expected, actualResults);
    }

    [TestMethod]
    public void Two_Digits_Yield_Nine_Representations()
    {
        var sut = "22";

        var expected = new List { "aa", "ab", "ac", "ba", "bb", "bc", "ca", "cb", "cc" };
        var actualResults = LetterCombinations.FromPhoneNumber(sut);

        CollectionAssert.AreEqual(expected, actualResults);
    }

    [TestMethod]
    public void Three_Digits_Yield_ThirtyNine_Representations()
    {
        var sut = "222";

        var actualResults = LetterCombinations.FromPhoneNumber(sut);

        var possibleCombinations = Math.Pow(3,3); //27

        Assert.AreEqual(possibleCombinations, actualResults.Count);
    }
}