PAT1040

#include<iostream>#include<vector>#include<string>using namespace std;int main() {    string s;    cin >> s;    int size = s.size()-1, i = 0,count=0, numAT=0,numT=0;    for (int i = size; i >= 0; --i) {        if ('T' == s[i]) ++numT;                else if ('A' == s[i]) numAT += numT;                 else {                                 count += numAT;                         if (count >= 1000000007) count %= 1000000007;        }    }    cout << count;    return 0;}

注意要从后向前考虑。从前向后比较难以实现