Tukukkk

I'm working on problem 91 on Leetcode.com called Decode Ways and I've successfully managed to get a working recusive solution but it results in Time Limited Exceeded(TLE). I'm new to utilizing memoization and I've been unable to discover how to properly memoize my results at each resursive step and check if I've already solve the sub-problem

How do I properly store my calculated results to 'evolve' my code from recursion to DP?

Prompt:

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

'A' -> 1

'B' -> 2

...

'Z' -> 26

Given a non-empty string containing only digits, determine the total number of ways to decode it.

Working Code (TLE):

public int NumDecodings(string s) 

{

    if (s == null || s.Length == 0) return 0;

    // int dp = new int[s.Length + 1];

    // for (int i = 0; i < dp.Length; ++i) dp[i] = -1;

    // dp[0] = 1;

    // dp[1] = Decode(s, 0, 1, dp) + Decode(s, 0, 2, dp);

    int jump1 = Decode(s, 0, 1);

    int jump2 = Decode(s, 0, 2);

    return jump1 + jump2;

    // return dp[1];

}



public int Decode(string s, int i, int len)

{

    if (i + len - 1 >= s.Length)

        return 0;



    // if (dp[i] > -1)

    //     return dp[i];



    int sub = Convert.ToInt32(s.Substring(i, len));

    if (sub > 26 || sub < 1 || s.Substring(i, len)[0] == '0') 

        return 0;

    else if (i + len - 1 == s.Length - 1 && sub < 27 && sub > 0)

        return 1;



    int jump1 = i + len - 1 + 1 < s.Length ? Decode(s, i + len, 1) : 0;

    int jump2 = i + len - 1 + 2 < s.Length ? Decode(s, i + len, 2) : 0;

    // dp[i] = jump1 + jump2;



    return jump1 + jump2;

    // return dp[i];

}