Leetcode#91 Decode Ways

原题地址

动态规划题，注意0导致的小陷阱。

代码：

int numDecodings(string s) {
        if (s.empty() || s[0] < '1' || s[0] > '9') return 0;
        
        int sum = s[s.length() - 1] >= '1' && s[s.length() - 1] <= '9' ? 1 : 0;
        int nextnext = 1;
        int next = sum;
        
        for (int i = s.length() - 2; i >= 0; i--) {
            if (s[i] >= '0' && s[i] <= '9') {
                if (s[i] == '1' || (s[i] == '2' && s[i + 1] >= '0' && s[i + 1] <= '6'))
                    sum = next + nextnext;
                else if (s[i] == '0')
                    sum = 0;
                else
                    sum = next;
                nextnext = next;
                next = sum;
            }
            else
                return 0;
        }
        
        return sum;
}