PAT A 1093 递推

/*
    递推，A前面有多少个P，后面有多少个T，把个数相乘就是以这个A为中心，pat的个数
*/
#include<cstdio>
#include<cstring>
using namespace std;
const int N =1e5;
const int mod = 1e9+7;
int main()
{
    int ans = 0;
    char str[N+5];
    int pnum[N+5] = {0};
    scanf("%s",str);
    int len = strlen(str);
    for(int i = 0; i<len; ++i)
    {
        if(i>0)
            pnum[i] = pnum[i-1];
        if(str[i] == 'P')
            pnum[i]++;

    }
    int tnum = 0;
    for(int i = len-1; i>=1; i--)
    {
        if(str[i]=='A')
            ans = (ans+pnum[i]*tnum)%mod;
        else if(str[i]=='T')
            tnum++;
    }
    printf("%d\n",ans);
    return 0;
}