Decode Ways

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.

Solution: dp. Note that '0' must be decoded in together with the number in front of it.

class Solution {
public:
    int numDecodings(string s) {
        if (s.empty() || s[0] == '0') 
            return 0;
        int N = s.size();
        int dp[N+1];
        dp[0] = 1;
        dp[1] = 1;
        for (int i = 1; i < N; ++i)
        {
            if (s[i] == '0')
            {
                if (s[i-1] != '1' && s[i-1] != '2')
                    return 0;
                dp[i+1] = dp[i-1];
            }
            else
            {
                int num = s[i] - '0' + (s[i-1] - '0') * 10;
                if (num >= 11 && num <= 26)
                    dp[i+1] = dp[i] + dp[i-1];
                else
                    dp[i+1] = dp[i];
            }
        }
        return dp[N];
    }
};