Crazy Diamond

题意翻译

有一个$1$到$n$的排列$p$，$n$是偶数 每次可以选择两个数$i,j$使$2\cdot|i-j|\ge n$，并交换$p_i,p_j$ 请用不超过$5n$次操作排序$p$，保证有解

题目描述

You are given a permutation $ p $ of integers from $ 1 $ to $ n $ , where $ n $ is an even number. Your goal is to sort the permutation. To do so, you can perform zero or more operations of the following type: - take two indices $ i $ and $ j $ such that $ 2 \cdot |i - j| \geq n $ and swap $ p_i $ and $ p_j $ . There is no need to minimize the number of operations, however you should use no more than $ 5 \cdot n $ operations. One can show that it is always possible to do that.

输入输出格式

输入格式

The first line contains a single integer $ n $ ( $ 2 \leq n \leq 3 \cdot 10^5 $ , $ n $ is even) — the length of the permutation. The second line contains $ n $ distinct integers $ p_1, p_2, \ldots, p_n $ ( $ 1 \le p_i \le n $ ) — the given permutation.

输出格式

On the first line print $ m $ ( $ 0 \le m \le 5 \cdot n $ ) — the number of swaps to perform. Each of the following $ m $ lines should contain integers $ a_i, b_i $ ( $ 1 \le a_i, b_i \le n $ , $ |a_i - b_i| \ge \frac{n}{2} $ ) — the indices that should be swapped in the corresponding swap. Note that there is no need to minimize the number of operations. We can show that an answer always exists.

输入输出样例

输入样例 #1

2
2 1

输出样例 #1

1
1 2

输入样例 #2

4
3 4 1 2

输出样例 #2

4
1 4
1 4
1 3
2 4

输入样例 #3

6
2 5 3 1 4 6

输出样例 #3

3
1 5
2 5
1 4

说明

In the first example, when one swap elements on positions $ 1 $ and $ 2 $ , the array becomes sorted. In the second example, pay attention that there is no need to minimize number of swaps. In the third example, after swapping elements on positions $ 1 $ and $ 5 $ the array becomes: $ [4, 5, 3, 1, 2, 6] $ . After swapping elements on positions $ 2 $ and $ 5 $ the array becomes $ [4, 2, 3, 1, 5, 6] $ and finally after swapping elements on positions $ 1 $ and $ 4 $ the array becomes sorted: $ [1, 2, 3, 4, 5, 6] $ .

题意翻译

有一个$1$到$n$的排列$p$，$n$是偶数 每次可以选择两个数$i,j$使$2\cdot|i-j|\ge n$，并交换$p_i,p_j$ 请用不超过$5n$次操作排序$p$，保证有解