Decode Ways [Leetcode]

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: dynamic programming; pay attention to the erroneous cases, like "....00...", or "...40...".

class Solution {
public:
    int numDecodings(string s) {
        // Note: The Solution object is instantiated only once and is reused by each test case.
        int n = s.size();
        if(n==0) return 0;
        vector<int> num(n, 1);
        //basic case
        if(s[0]=='0')
            return 0;
        for(int i=1; i<n; i++){
            if(s[i]=='0' && s[i-1]=='0')
                return 0;
            if(s[i-1]=='0')
                num[i]=num[i-1];
            else if(s[i]=='0'){
                if(s[i-1]>'2') 
                    return 0;
                num[i]=(i==1?1:num[i-2]);
            }
            else if(10*(s[i-1]-'0')+(s[i]-'0')<=26)
                num[i]=num[i-1]+(i==1?1:num[i-2]);
            else
                num[i]=num[i-1];
        }
        return num.back();
    }
};