P7999 [WFOI - 01] Flipping Sequence (requese)

Simplified statement: [$\texttt{Link}$](https://www.luogu.com.cn/paste/kmmn8pyd)。 Why don’t you go solve [this problem](https://www.luogu.com.cn/problem/P8223) after finishing this one?

You need to sort a permutation of $1\sim n$ on a strange computer. You may choose a number $x$. Then each time, you may reverse a segment of length $x+1$ or a segment of length $x-1$. Please restore the sequence to $1\sim n$ within $20\times n$ operations. (Setter’s note: The current best can be under 15000 operations. Please try to optimize your algorithm.)

The input consists of $2$ lines. The first line contains an integer $n$. The second line contains $n$ integers, representing the sequence $a$.

The output consists of $m + 2$ lines. The first two lines each contain $1$ integer, which are $x,m$ respectively. Here, $m$ is the number of operations. The next $m$ lines each contain two integers, representing the left and right endpoints of the reversed interval. This problem uses $\text{SPJ}$, and you will be judged as long as the reverse operations are correct.

- **Sample $1$ explanation:** Reverse $(1,2)$, and the sequence becomes $1,2$. - **Sample $2$ explanation:** Reverse $(1,5)$, and the sequence becomes $1,4,3,2,5$. Reverse $(2,4)$, and the sequence becomes $1,2,3,4,5$. **This problem uses bundled Subtask tests.** Subtask ID | Constraints and Notes :-: | :-: **Subtask #0** ($\texttt{1 pts}$) | $n=1$ **Subtask #1** ($\texttt{2 pts}$) | $n=2$ **Subtask #2** ($\texttt{3 pts}$) | $n=3$ **Subtask #3** ($\texttt{4 pts}$) | $n=4$ **Subtask #4** ($\texttt{20 pts}$) | $1\le n\le 50$ **Subtask #5** ($\texttt{20 pts}$) | $1\le n\le 100$ **Subtask #6** ($\texttt{50 pts}$) | $1\le n\le 10^3$ For $100\%$ of the testdata, $1\le n,a_i\le 10^3$, and the data guarantees that $a$ is a permutation of $1\sim n$. Translated by ChatGPT 5