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

    }
};