class Solution {
public:
    int numDecodings(string s) {
        int n = s.size();
        s = ' ' + s;//为了方便，让下标从1开始，所以加上一个空格
        vector<int> f(n + 1);
        f[0] = 1;//0个数字是的解码方案算作1
        for (int i = 1; i <= n; i ++ ) {
            if (s[i] >= '1' && s[i] <= '9') f[i] += f[i - 1];
            if (i > 1) {
                int t = (s[i - 1] - '0') * 10 + (s[i] -'0');
                if (t >= 10 && t <= 26) f[i] += f[i - 2];
            }
        }
        return f[n];
    }
};