Codeforces 1238 D:AB-string

题目传送门： AB-string
D. AB-string
time limit per test2 seconds
memory limit per test256 megabytes
The string t1 t2…tk is good if each letter of this string belongs to at least one palindrome of length greater than 1.
A palindrome is a string that reads the same backward as forward. For example, the strings A, BAB, ABBA, BAABBBAAB are palindromes, but the strings AB, ABBBAA, BBBA are not.

Here are some examples of good strings:

t = AABBB (letters t1, t2 belong to palindrome t1…t2 and letters t3, t4, t5 belong to palindrome t3…t5);
t = ABAA (letters t1, t2, t3 belong to palindrome t1…t3 and letter t4 belongs to palindrome t3…t4);
t = AAAAA (all letters belong to palindrome t1…t5);
You are given a string s of length n, consisting of only letters A and B.

You have to calculate the number of good substrings of string s.

Input
The first line contains one integer n (1≤n≤3⋅105) — the length of the string s.

The second line contains the string s, consisting of letters A and B.

Output
Print one integer — the number of good substrings of string

题意：含有长度大于1的回文串的字串即为好的字符串，要求求出给定字符串所有好的字符串。
思路： 逆向思维，题目要求求好的字符串，只需求出非好的字符串然后所有长度大于二的字符串的减去非好的字符串即可。题目要求含有长度大于1的回文串即为好的字符串，而且字符串中只有A，B两个字母，所以只有AAAAB或者BAAAA或者ABBBB或者BBBBA这种形式字符串是非好的,其他的字串都是好的。所以求出这些字符串的数量即可。

#include 
#include 
#include 
#include 

using namespace std;
typedef long long ll;
const int maxx=1e6+7;
char s[maxx];
int main()
{
    ll n,cnt=0,last=0,ans=0;
    scanf("%I64d",&n);//输入
    scanf("%s",s);
    for(int i=0;i