1093 Count PAT's（25 分）

The string APPAPT contains two PAT's as substrings. The first one is formed by the 2nd, the 4th, and the 6th characters, and the second one is formed by the 3rd, the 4th, and the 6th characters.

Now given any string, you are supposed to tell the number of PAT

Input Specification:

Each input file contains one test case. For each case, there is only one line giving a string of no more than 10​5​​ characters containing only P, A, or T.

Output Specification:

For each test case, print in one line the number of PAT

Sample Input:

APPAPT

Sample Output:

2

主要就是利用累加的思想，這樣不容易超時。

#include <bits/stdc++.h>
using namespace std;
const int maxn = 100000 + 10;
const int MOD = 1000000007;
int main() {
    string cas;
    cin >> cas;
    int ans = 0;
    int ansp = 0, ansa = 0;
    for (int i = 0; i < cas.length(); i++) {
        if (cas[i] == 'P') {
            ansp++;
        }
        else if (cas[i] == 'A') {
            ansa = (ansa + ansp)%MOD;
        }
        else {
            ans = (ans + ansa)%MOD;
        }
    }
    cout << ans%MOD << endl;
    return 0;
}