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Minimum number of steps 805D

http://codeforces.com/contest/805/problem/D

 

 

D. Minimum number of steps
time limit per test
1 second
memory limit per test
256 megabytes
input
standard input
output
standard output

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 modulo109 + 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 from1 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".

 

题意:将字符串中ab 替换成 bba 进行多少次操作字符串中没有 ab

分析:将字符ab 替换成 bba 可以看成 a 向右移动一位, ab 中的 b 后增加一个 b

    每次将a 字符移动到所有b字符 的右端, 下一个a右边的b就多了一倍, 所以每遇到一个a 加上右边的b,然后更新右边的b为2*b

 1 #include <bits/stdc++.h>
 2 using namespace std;
 3 #define ll long long
 4 
 5 const ll mod = 1e9 + 7;
 6 // "ab" ?→? "bba".
 7 // "aab" ?→? "abba" ?→? "bbaba"?→? "bbbbaa".
 8 
 9 int main(){
10     string str;
11     while(cin >> str){
12         //cout << str << endl;
13         ll b = 0;
14         ll ans = 0;
15         for(ll i = str.length() - 1; i >= 0; i--){
16             // cout << "b " << b<< endl;
17             if(str[i] == 'b')
18                 b++;
19             //cout << "b " << b<< endl;
20             if(str[i] == 'a'){
21                 ans += b%mod;
22                 //cout << ans << endl;
23                 ans %= mod;
24                 b *= 2;
25                 b %= mod;
26             }
27         }
28         cout << ans% mod << endl;
29     }
30 }

 

 

 

posted on 2017-05-07 23:00  夏晴天  阅读(251)  评论(0编辑  收藏

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