Simply try building a recursive solution first and then change it to iterative one.
This problem is recursive and can be broken into sub-problems. We start from the end of the given digit sequence. We initialize the total count of decodings as 0. We recur for two subproblems.
1) If the last digit is non-zero, recur for remaining (n-1) digits and add the result to the total count.
2) If...