Decode Ways

class Solution {
public:
    int numDecodings(string s) {
        int len = s.length();
        if (len == 0 || s[0] == ‘0‘) return 0;
        vector<int> ret(len + 1, 0);
        ret[0] = ret[1] = 1;
        for (int i = 2; i <= len; ++i) {
            if (s[i - 2] == ‘1‘ || (s[i - 2] == ‘2‘ && s[i - 1] <= ‘6‘)) {
                ret[i] += ret[i - 2];
            } 
            
            if (s[i - 1] != ‘0‘) ret[i] += ret[i - 1];
        }
        
        return ret[len];
    }
    

};