using System;

public class Program

{

    public static void Main()

    {

        /** A message containing letters from A-Z is being encoded to numbers using the following mapping:

        'A' -> 1

        'B' -> 2

        ...

        'Z' -> 26

        Input: "12"

        Output: 2

        Explanation: It could be decoded as "AB" (1 2) or "L" (12).

        Input: "226"

        Output: 3

        Explanation: It could be decoded as "BZ" (2 26), "VF" (22 6), or "BBF" (2 2 6).**/

        String s = "12425";

        Console.WriteLine(numDecodings(s));

        Console.ReadLine();

    }

    //Find the diffrent way of coding a letter. 

    public static int numDecodings(String s)

    {

        int n = s.Length;

        int[] dp = new int[n + 1];

        dp[0] = 1;

        dp[1] = s[1] != '0' ? 1 : 0;

        for (int i = 2; i <= s.Length; i++)

        {

            int first = Int32.Parse(s.Substring(i - 1, 1));

            int second = Int32.Parse(s.Substring(i - 2, 2));

            if (first >= 0 && first <= 9)

            {

                dp[i] += dp[i - 1];

            }

            if (second >= 10 && second <= 26)

            {

                dp[i] += dp[i - 2];

            }

        }

​

        return dp[n];

    }

}