LeetCode 0091 - Decode Ways

'A' -> 1
'B' -> 2
...
'Z' -> 26

class Solution {
public:
    int numDecodings(string s) {
        if(s.empty() || s[0] == '0')
            return 0;
        int r1 = 1;
        int r2 = 1;
        for(int i = 1; s[i]; i++) {
            if(s[i] == '0')
                r1 = 0;
            if(s[i-1] == '1' || (s[i-1] == '2' && s[i] <= '6'))
                r1 += r2, r2 = r1 - r2;
            else
                r2 = r1;
        }
        return r1;
    }
};