public void Test_FindMissingNumber(string input, int expected)
        {
            var result = NumberUtilities.FindMissingNumber(input);

            Assert.That(result, Is.EqualTo(expected));

            var query = input
                        .Split(' ')
                        .Select(Int32.Parse)
                        .GroupBy(s => s)
                        .OrderBy(s => s.Count())
                        .Select(x => new
            {
                Number          = x.Key,
                RepetitionCount = x.Count()
            });

            foreach (var item in query)
            {
                Console.WriteLine($"Number: {item.Number}, repetitionCount: {item.RepetitionCount}");
            }
        }