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[] < '' || s[] > '') return ;

         int sum = s[s.length() - ] >= '' && s[s.length() - ] <= '' ?  : ;
         int nextnext = ;
         int next = sum;

         for (int i = s.length() - ; i >= ; i--) {
             if (s[i] >= '' && s[i] <= '') {
                 if (s[i] == '' || (s[i] == '' && s[i + ] >= '' && s[i + ] <= ''))
                     sum = next + nextnext;
                 else if (s[i] == '')
                     sum = ;
                 else
                     sum = next;
                 nextnext = next;
                 next = sum;
             }
             else
                 return ;
         }

         return sum;
 }