CF804B Minimum number of steps

思路：

找规律。参考了http://blog.csdn.net/harlow_cheng/article/details/71190999。

实现：

 #include <iostream>
 using namespace std;
 const int mod = 1e9 + ;
 int main()
 {
     string s;
     cin >> s;
     int n = s.length();
     int cnt = ;
     int sum = ;
     for (int i = ; i < n; i++)
     {
         if (s[i] == 'a') cnt = cnt *  % mod;
         else sum = (sum + cnt - ) % mod;
     }
     cout << sum << endl;
     return ;
 }