[Leetcode] Decode Ways

Problem

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

'A' -> 1

'B' -> 2

...

'Z' -> 26

Given an encoded message containing digits, determine the total number of ways to decode it.

For example,
Given encoded message "12", it could be decoded as "AB" (1 2) or "L" (12).

The number of ways decoding "12" is 2.

Algorithm

Let num[i] = number of ways to decode the string ending at S[i]. 

         num[i]  =  num[i-1]                            if S[i] has valid decoding

                        num[i-2] + num[i-1]            if both S[i] and S[i-1 to i] have valid decoding 

If 1<= S[i] <= 9, it is  valid

If S[i-1] = 1, 0 <= S[i] <= 9, it is valid

If S[i-1] = 2, 0 <= S[i] <= 6, it is valid

Others are not valid

Boundary condition, L[0] = 0, if S[0] = 0 

Code