//第二题---更好理解

	int count_seq(string str, int low, int high) {



	 int result = 0;



	 if (high - low == 0) {
  return 1;
 }



	 if (high - low == 1) {
  if (str[high] == str[low])
   return 3;
  else
   return 2;
 }



	 if (str[low] == str[high]) {
  result = count_seq(str, low + 1, high) + count_seq(str, low, high - 1) + 1;
 }
 else {
  result = count_seq(str, low + 1, high) + count_seq(str, low, high - 1) - count_seq(str, low + 1, high - 1);
 }



	 return result;
}



	int main() {
 string str = "ABA";
 cout << count_seq(str, 0, str.length() - 1) << endl;
}