Count Possible Decoding

README.txt

Given the mapping a = 1, b = 2, ... z = 26, and an encoded message, 
count the number of ways it can be decoded.

For example, the message '111' would give 3, since it could be decoded as 'aaa', 'ka', and 'ak'.

You can assume that the messages are decodable. For example, '001' is not allowed.

Program.cs

class Program
{
   static readonly Dictionary<string, long> memoized = new Dictionary<string, long>();
   static void Main(string[] args)
   {
      Console.WriteLine(CountDecode("111"));
   }

   private static long CountDecode(string s)
   {
      if (string.IsNullOrWhiteSpace(s))
      {
          return 1L;
      }
      if (memoized.ContainsKey(s))
      {
          return memoized[s];
      }
      memoized.Add(s, CountDecode(s.Substring(1)));
      if (s.Length > 1 && int.Parse(s.Substring(0, 2)) <= 26)
      {
          memoized[s] = memoized[s] + CountDecode(s.Substring(2));
      }
      return memoized[s];
   }
}