Codeforces Round #411 D. Minimum number of steps （贪心。）

We have a string of letters ‘a’ and ‘b’. We want to perform some operations on it. On each step we choose one of substrings “ab” in the string and replace it with the string “bba”. If we have no “ab” as a substring, our job is done. Print the minimum number of steps we should perform to make our job done modulo 109 + 7.

The string “ab” appears as a substring if there is a letter ‘b’ right after the letter ‘a’ somewhere in the string.
Input

The first line contains the initial string consisting of letters ‘a’ and ‘b’ only with length from 1 to 106.
Output

Print the minimum number of steps modulo 109 + 7.
Examples
input

ab

output

1

input

aab

output

3

Note

The first example: “ab”  →  “bba”.

The second example: “aab”  →  “abba”  →  “bbaba”  →  “bbbbaa”.

参考http://blog.csdn.net/dormousenone/article/details/71190792
**#解题思路
根据题意，转换规则如下：
ab→bba
abb→bbbba
abbb→bbbbbba**

#include<bits/stdc++.h>using namespace std;const int mod = 1e9 + 7;char s[1000010];int main(){    scanf(" %s",s);    int len = strlen(s);    long long cntb = 0, ans = 0;    for(int i=len-1;i>=0;--i)    {        if(s[i] == 'a') {            (ans += cntb) %= mod;            (cntb *= 2) %= mod;        } else {            cntb++;        }    }    printf("%I64d\n", ans%mod);}