CF2084C You Soared Afar With Grace

You are given a permutation $ a $ and $ b $ of length $ n $ $ ^{\text{∗}} $ . You can perform the following operation at most $ n $ times: - Choose two indices $ i $ and $ j $ ( $ 1 \le i, j \le n $ , $ i \ne j $ ), swap $ a_i $ with $ a_j $ , swap $ b_i $ with $ b_j $ . Determine whether $ a $ and $ b $ can be reverses of each other after operations. In other words, for each $ i = 1, 2, \ldots, n $ , $ a_i = b_{n + 1 - i} $ . If it is possible, output any valid sequence of operations. Otherwise, output $ -1 $ . $ ^{\text{∗}} $ A permutation of length $ n $ is an array consisting of $ n $ distinct integers from $ 1 $ to $ n $ in arbitrary order. For example, $ [2,3,1,5,4] $ is a permutation, but $ [1,2,2] $ is not a permutation ( $ 2 $ appears twice in the array), and $ [1,3,4] $ is also not a permutation ( $ n=3 $ but there is $ 4 $ in the array).

Each test contains multiple test cases. The first line contains the number of test cases $ t $ ( $ 1 \le t \le 10^4 $ ). The description of the test cases follows. The first line of each test case contains a single integer $ n $ ( $ 2 \le n \le 2 \cdot 10^5 $ ) — the length of the permutations. The second line contains $ n $ integers $ a_1, a_2, \ldots, a_n $ ( $ 1 \le a_i \le n $ ). The third line contains $ n $ integers $ b_1, b_2, \ldots, b_n $ ( $ 1 \le b_i \le n $ ). It is guaranteed that $ a $ and $ b $ are permutations of length $ n $ . It is guaranteed that the sum of $ n $ over all test cases does not exceed $ 2\cdot 10^5 $ .

For each test case, if it is impossible, output $ -1 $ in the only line. Otherwise, output a single integer $ m $ ( $ 0 \le m \le n $ ) — the number of operations in the first line. In the following $ m $ lines, output two integers — the indices $ i $ and $ j $ ( $ 1 \le i, j \le n $ , $ i \ne j $ ) in each operation in order. If there are multiple solutions, print any of them.

In the second test case, $ b $ is already the reverse of $ a $ . In the third test case, after performing the following operation, $ b $ will become the reverse of $ a $ : - Swap $ a_1, a_2 $ and swap $ b_1, b_2 $ . Now $ a = [3, 1, 2, 4] $ and $ b = [4, 2, 1, 3] $ . In the fourth test case, after performing the following operations in order, $ b $ will become the reverse of $ a $ : - Swap $ a_1, a_2 $ and swap $ b_1, b_2 $ . Now $ a = [5, 2, 1, 3, 4] $ and $ b = [5, 3, 4, 2, 1] $ . - Swap $ a_1, a_3 $ and swap $ b_1, b_3 $ . Now $ a = [1, 2, 5, 3, 4] $ and $ b = [4, 3, 5, 2, 1] $ .