Decode Ways

<-M 91> Decode Ways

class Solution {
public:
    int numDecodings(string s) {
        if(s.empty() || s[0] == '0')
            return 0;
        int prev = 0;
        int cur = 1;

        for(int i = 1; i <= s.size(); i++) {
            if(s[i - 1] == '0')
                cur = 0;

            if(i < 2 || !(s[i - 2] == '1' || (s[i - 2] == '2' && s[i - 1] <= '6')))
                prev = 0;

            int temp = cur;
            cur = prev + cur;
            prev = temp;
        }
        return cur;
    }
};