Minimum number of steps
题意翻译
题面：
你有一串字符串,仅由a,b组成,一次操作为"ab"->"bba",求使原串中没有a在b前面的操作次数.
输入格式：
仅一行：
一串长为l的字符串(只由a,b组成).
输出格式：
一个整数,即操作次数(%1e9+7).
数据范围：
l:[1,1e6]
注意：
None.
翻译贡献者：尘染梦
题目描述
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 $ 10^{9}+7 $ .
The string "ab" appears as a substring if there is a letter 'b' right after the letter 'a' somewhere in the string.
输入输出格式
输入格式
The first line contains the initial string consisting of letters 'a' and 'b' only with length from $ 1 $ to $ 10^{6} $ .
输出格式
Print the minimum number of steps modulo $ 10^{9}+7 $ .
输入输出样例
输入样例 #1
ab
输出样例 #1
1
输入样例 #2
aab
输出样例 #2
3
说明
The first example: "ab" $ → $ "bba".
The second example: "aab" $ → $ "abba" $ → $ "bbaba" $ → $ "bbbbaa".