Problem

I want to explain the brute force for F.

In the tutorial of F,it said "however, deriving them manually is quite complicated, as there are many situations."

But I consider this problem as a perfect chance to practice case work:

Do the first three letters by case work.

First, whether $$$s_1$$$ is 'a' or 'b' doesn't matter.

We can just record the longest 'ababab' prefix and the value of $$$s_{mid}$$$ ,and do case work. Do two letters $$$x,y$$$ once.

1. p=1; p is odd number ;p is even number
2. $$$s_1=s_{mid}$$$,$$$s_1\neq s_{mid}$$$
3. $$$p=mid-1$$$,$$$p\neq mid-1$$$
4. $$$x=a$$$,$$$x=b$$$
5. $$$y=a$$$,$$$y=b$$$

In the $$$p=1$$$,we don't care $$$p=mid-1$$$.

As in the code,there is $$$2\times 2 \times 2+2\times 2\times 2\times 2\times 2=40$$$ cases,handle them one by one and get c++ code in 15kb.

I highly recommend you reading code, and I'm glad you to find my mistakes.