Codeforces•Sep 27, 2025
Minimum number of steps
Hazrat Ali
Codeforces
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
Solition
#include <bits/stdc++.h>
#define P 1000000007
using namespace std;
long long fast_pow(long long x, long long y) {
if (y == 0) {
return 1;
}
if (y == 1) {
return x;
}
if (y % 2 == 0) {
return fast_pow(x * x % P, y / 2) % P;
}
return (x % P * fast_pow(x * x % P, (y - 1) / 2) % P) % P;
}
int main() {
string s;
cin >> s;
long long ans = 0, cnt = 0;
for (int i = 0; i < s.size(); i++) {
if (s[i] == 'a') {
cnt++;
} else {
ans += fast_pow(2, cnt) - 1;
ans %= P;
}
}
cout << ans << endl;
return 0;
}