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CF1867B.XOR Palindromes
XOR Palindromes
You are given a binary string of length (a string that consists only of and ). A number is good if there exists a binary string of length , containing ones, such that if each symbol is replaced by (where denotes the bitwise XOR operation), then the string becomes a palindrome.
You need to output a binary string of length , where () is equal to if number is good, and otherwise.
A palindrome is a string that reads the same from left to right as from right to left. For example, 01010, 1111, 0110 are palindromes.
Input
Each test contains multiple test cases. The first line contains the number of test cases (). The description of the test cases follows.
The first line of each test case contains a single integer ().
The second line of each test case contains a binary string of length .
It is guaranteed that the sum of over all test cases does not exceed .
Output
For each test case, output a single line containing a binary string of length - the answer to the problem.
Note
Consider the first example.
- because we can choose
010100, then the string becomes111111, which is a palindrome. - because we can choose
101011. - It can be shown that for all other , there is no answer, so the remaining symbols are .
Samples
5
6
101011
5
00000
9
100100011
3
100
1
1
0010100
111111
0011111100
0110
11
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