Serval and Shift-Shift-Shift
Contest: Codeforces Round 853 (Div. 2)
Difficulty rating: 2200
Tags: bitmasks, brute force, constructive algorithms, implementation

Serval has two $$$n$$$-bit binary integer numbers $$$a$$$ and $$$b$$$. He wants to share those numbers with Toxel.

Since Toxel likes the number $$$b$$$ more, Serval decides to change $$$a$$$ into $$$b$$$ by some (possibly zero) operations. In an operation, Serval can choose any positive integer $$$k$$$ between $$$1$$$ and $$$n$$$, and change $$$a$$$ into one of the following number:

- $$$a\oplus(a\ll k)$$$
- $$$a\oplus(a\gg k)$$$

In other words, the operation moves every bit of $$$a$$$ left or right by $$$k$$$ positions, where the overflowed bits are removed, and the missing bits are padded with $$$0$$$. The bitwise XOR of the shift result and the original $$$a$$$ is assigned back to $$$a$$$.

Serval does not have much time. He wants to perform no more than $$$n$$$ operations to change $$$a$$$ into $$$b$$$. Please help him to find out an operation sequence, or determine that it is impossible to change $$$a$$$ into $$$b$$$ in at most $$$n$$$ operations. You do not need to minimize the number of operations.

In this problem, $$$x\oplus y$$$ denotes the bitwise XOR operation of $$$x$$$ and $$$y$$$. $$$a\ll k$$$ and $$$a\gg k$$$ denote the logical left shift and logical right shift.

Input format:
Each test contains multiple test cases. The first line contains the number of test cases $$$t$$$ ($$$1\le t\le2\cdot10^{3}$$$). The description of the test cases follows.

The first line of each test case contains a single integer $$$n$$$ ($$$1\le n\le2\cdot10^{3}$$$) — the number of bits in numbers $$$a$$$ and $$$b$$$.

The second and the third line of each test case contain a binary string of length $$$n$$$, representing $$$a$$$ and $$$b$$$, respectively. The strings contain only characters 0 and 1.

It is guaranteed that the sum of $$$n$$$ over all test cases does not exceed $$$2\cdot10^{3}$$$.

Output format:
For each test case, if it is impossible to change $$$a$$$ into $$$b$$$ in at most $$$n$$$ operations, print a single integer $$$-1$$$.

Otherwise, in the first line, print the number of operations $$$m$$$ ($$$0\le m\le n$$$).

If $$$m>0$$$, in the second line, print $$$m$$$ integers $$$k_{1},k_{2},\dots,k_{m}$$$ representing the operations. If $$$1\le k_{i}\le n$$$, it means logical left shift $$$a$$$ by $$$k_{i}$$$ positions. If $$$-n\le k_{i}\le-1$$$, it means logical right shift $$$a$$$ by $$$-k_{i}$$$ positions.

If there are multiple solutions, print any of them.

Constraints:
Time limit: 2.0 s
Memory limit: 256.0 MB

Note:
In the first test case:

The first operation changes $$$a$$$ into $$$\require{cancel}00111\oplus\cancel{001}11\underline{000}=11111$$$.

The second operation changes $$$a$$$ into $$$\require{cancel}11111\oplus\underline{00}111\cancel{11}=11000$$$.

The bits with strikethroughs are overflowed bits that are removed. The bits with underline are padded bits.

In the second test case, $$$a$$$ is already equal to $$$b$$$, so no operations are needed.

In the third test case, it can be shown that $$$a$$$ cannot be changed into $$$b$$$.

Example input:
3
5
00111
11000
1
1
1
3
001
000

Example output:
2
3 -2
0
-1

Editorial: