91. Decode Ways

class Solution {
public:
    int numDecodings(string s) {
        int n = s.size();
        vector<int> f(n+1);
        f[0] = 1;
        for(int i=1; i<=n; ++i)
        {
            if(s[i-1]!='0')
            {
                f[i] += f[i-1];
            }
            if(i>1 && s[i-2]!='0' && (s[i-2]-'0')*10 + (s[i-1]-'0') <= 26)
            {
                f[i] += f[i-2];
            }
        }

        return f[n];
    }
};