【思维】ABC

题目描述

You are given a string s consisting of A, B and C.
Snuke wants to perform the following operation on s as many times as possible:
Choose a contiguous substring of s that reads ABC and replace it with BCA.
Find the maximum possible number of operations.
Constraints
1≤|s|≤200000
Each character of s is A, B and C.

输入

Input is given from Standard Input in the following format:
S

输出

Find the maximum possible number of operations.

样例输入

ABCABC

样例输出

3

提示

You can perform the operations three times as follows: ABCABC → BCAABC → BCABCA → BCBCAA. This is the maximum result.

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参考：SZG大佬的题解

题意

在一个只包含A、B、C的字符串，有一种操作，可使 “ABC” 变成 ”BCA“，求字符串s的最多操作数。

1≤∣s∣≤200000

思路

  易得，该操作是将A与BC交换位置，可用 1、0分别代表“A”、“BC”。题意转化对一个只包含10的序列，

将所有的10更新01，即将所有的0放在1前面。假设序列中共有kk个0，每个0前面有ai​个1，则ans=∑​ai （1,k）​

  对于单独B、C，则可看作是两个序列分隔的标志。

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 #include<bits/stdc++.h>
 using namespace std;
 const int N = 2e5+;
 typedef long long ll;
 char s[N];
 int main()
 {
     scanf("%s",s);
     ll cnt =  ;
     ll ans =  , i =  ;
     int len = strlen(s);
     while ( s[i] ) {
         if( s[i] == 'A' ){
             //printf("#1 %d \n" ,i);
             cnt ++ ;
             i++ ;
         }else if ( s[i] == 'B' && s[i+] == 'C' ){
             //printf("#2 %d \n" ,i);
             ans = ans + cnt ;
             i+= ;
             if( i >= len ) break ;
         }else if ( s[i] == 'B' || s[i] == 'C' || s[i] == '\0' ){
             cnt =  ;
             i++ ;
         }
     }
     //ans = ans + cnt ;
     printf("%lld\n",ans);
 }

ABC