A QAQ
“QAQ” is a word to denote an expression of crying. Imagine “Q” as eyes with tears and “A” as a mouth.Now Diamond has given Bort a string consisting of only uppercase English letters of length n. There is a great number of “QAQ” in the string (Diamond is so cute!). illustration by 貓屋 Bort wants to know how many subsequences “QAQ” are in the string Diamond has given. Note that the letters “QAQ” don’t have to be consecutive, but the order of letters should be exact.
Input
The only line contains a string of length n (1 ≤ n ≤ 100). It’s guaranteed that the string only contains uppercase English letters.
Output
Print a single integer — the number of subsequences “QAQ” in the string.
輸入
QAQAQYSYIOIWIN
輸出
4
這道題一般很容易想到的是三重for循環直接暴力，數據量是100之內，三重頂多1e6所以能過，但是如果數據量達到10000不能用三重for循環該怎麼寫呢，下面直接看代碼，原理自己領悟；
#include<bits/stdc++.h>
using namespace std;
string str;
int main()
{
cin>>str;
int now = 0 , a = 0, y = 0;
for(int i = 0; i < str.size() ;i++ )
{
if ( str[i] == 'Q' )
{
a ++ ;
now +=y;
}
if ( str[i] == 'A' )
{
y += a;
}
}
cout<< now <<endl;
}