Minimal String Xoration

* 给定一个长度为 $2^n$，只包含小写字母的字符串 $s$。 * 你可以将字符串的下标全部异或一个 $[0,2^n)$ 的整数 $k$，即构造一个与 $s$ 等长的新字符串 $t$，使得 $t_i=s_{i\oplus k}$。 * 最小化 $t$ 的字典序，并输出字典序最小的 $t$。 * $n\leq 18$。

You are given an integer $ n $ and a string $ s $ consisting of $ 2^n $ lowercase letters of the English alphabet. The characters of the string $ s $ are $ s_0s_1s_2\cdots s_{2^n-1} $ . A string $ t $ of length $ 2^n $ (whose characters are denoted by $ t_0t_1t_2\cdots t_{2^n-1} $ ) is a xoration of $ s $ if there exists an integer $ j $ ( $ 0\le j \leq 2^n-1 $ ) such that, for each $ 0 \leq i \leq 2^n-1 $ , $ t_i = s_{i \oplus j} $ (where $ \oplus $ denotes the operation [bitwise XOR](https://en.wikipedia.org/wiki/Bitwise_operation#XOR)). Find the lexicographically minimal xoration of $ s $ . A string $ a $ is lexicographically smaller than a string $ b $ if and only if one of the following holds: - $ a $ is a prefix of $ b $ , but $ a \ne b $ ; - in the first position where $ a $ and $ b $ differ, the string $ a $ has a letter that appears earlier in the alphabet than the corresponding letter in $ b $ .

The first line contains a single integer $ n $ ( $ 1 \le n \le 18 $ ). The second line contains a string $ s $ consisting of $ 2^n $ lowercase letters of the English alphabet.

Print a single line containing the lexicographically minimal xoration of $ s $ .

2
acba

abca

3
bcbaaabb

aabbbcba

4
bdbcbccdbdbaaccd

abdbdccacbdbdccb

5
ccfcffccccccffcfcfccfffffcccccff

cccccffffcccccffccfcffcccccfffff

1
zz

zz

In the first test, the lexicographically minimal xoration $ t $ of $ s = $ "acba" is "abca". It's a xoration because, for $ j = 3 $ , - $ t_0 = s_{0 \oplus j} = s_3 = $ "a"; - $ t_1 = s_{1 \oplus j} = s_2 = $ "b"; - $ t_2 = s_{2 \oplus j} = s_1 = $ "c"; - $ t_3 = s_{3 \oplus j} = s_0 = $ "a". There isn't any xoration of $ s $ lexicographically smaller than "abca".In the second test, the minimal string xoration corresponds to choosing $ j = 4 $ in the definition of xoration. In the third test, the minimal string xoration corresponds to choosing $ j = 11 $ in the definition of xoration. In the fourth test, the minimal string xoration corresponds to choosing $ j = 10 $ in the definition of xoration. In the fifth test, the minimal string xoration corresponds to choosing either $ j = 0 $ or $ j = 1 $ in the definition of xoration.