A16440 | Parallel Swaps Sort

The first line contains a single integer $n$ ( $2 \le n \le 3 \cdot 10^5$ ) — the length of the permutation.

The second line contains $n$ integers $p_1, p_2, \ldots, p_n$ ( $1 \le p_i \le n$ , the $p_i$ are distinct) — the permutation before performing the operations.

Output your operations in the following format.

The first line should contain an integer $k$ ( $0 \le k \le 10^6$ ) — the number of operations.

The next $k$ lines represent the $k$ operations in order. Each of these $k$ lines should contain two integers $l$ and $r$ ( $1 \leq l < r \leq n$ , $r-l+1$ must be even) — the corresponding operation consists in choosing the subarray $[l, r]$ and swapping its elements according to the problem statement.

After all the operations, $a_i = i$ must be true for each $i$ ( $1 \leq i \leq n$ ).

In the first test:

- At the beginning, $p = [2, 5, 4, 1, 3]$ .
- In the first operation, you can choose $[l, r] = [1, 4]$ . Then, $(a_1, a_2)$ are swapped and $(a_3, a_4)$ are swapped. The new permutation is $p = [5, 2, 1, 4, 3]$ .
- In the second operation, you can choose $[l, r] = [1, 2]$ . Then, $(a_1, a_2)$ are swapped. The new permutation is $p = [2, 5, 1, 4, 3]$ .
- In the third operation, you can choose $[l, r] = [2, 5]$ . Then, $(a_2, a_3)$ are swapped and $(a_4, a_5)$ are swapped. The new permutation is $p = [2, 1, 5, 3, 4]$ .
- In the fourth operation, you can choose $[l, r] = [1, 4]$ . Then, $(a_1, a_2)$ are swapped and $(a_3, a_4)$ are swapped. The new permutation is $p = [1, 2, 3, 5, 4]$ .
- In the fifth operation, you can choose $[l, r] = [4, 5]$ . Then, $(a_4, a_5)$ are swapped. The new permutation is $p = [1, 2, 3, 4, 5]$ , which is sorted.

In the second test, the permutation is already sorted, so you do not need to perform any operation.