https://leetcode.com/problems/decode-ways/

decode-ways.cpp

class Solution {
public:
    int numDecodings(const string& s) {
        if (s.empty()) return 0;
        if (s.size() == 1) return s[0] != '0';
        int n_2 = s.back() != '0';
        int n_1 = (s[s.size()-2] != '0') * n_2 + (s[s.size()-2] == '1' || s[s.size()-2] == '2' && s.back() <= '6');
        for (int i = s.size()-3; i >= 0; --i) {
            int num = (s[i] > '0') * n_1 + (s[i] == '1' || s[i] == '2' && s[i+1] <= '6') *  n_2;
            n_2 = n_1;
            n_1 = num;
        }
        return n_1;
    }
};