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 + 7;
int main()
{
    string s;
    cin >> s;
    int n = s.length();
    int cnt = 1;
    int sum = 0;
    for (int i = 0; i < n; i++)
    {
        if (s[i] == 'a') cnt = cnt * 2 % mod;
        else sum = (sum + cnt - 1) % mod;
    }
    cout << sum << endl;
    return 0;
}