String Similarity

定义两个字符串$a,b$是相似的，当且仅当这两个字符串长度相同，且存在$i\in [1,|a|]$，使得$a_i=b_i$。 给定一个长度为$2n-1$的01字符串$S$，你需要构造出一个长度为$n$的字符串，使得它与$S$的每一个长度为$n$的子串都是相似的。 题目保证有解。 多测，$1\le n\le 50$。

A binary string is a string where each character is either 0 or 1. Two binary strings $ a $ and $ b $ of equal length are similar, if they have the same character in some position (there exists an integer $ i $ such that $ a_i = b_i $ ). For example: - 10010 and 01111 are similar (they have the same character in position $ 4 $ ); - 10010 and 11111 are similar; - 111 and 111 are similar; - 0110 and 1001 are not similar. You are given an integer $ n $ and a binary string $ s $ consisting of $ 2n-1 $ characters. Let's denote $ s[l..r] $ as the contiguous substring of $ s $ starting with $ l $ -th character and ending with $ r $ -th character (in other words, $ s[l..r] = s_l s_{l + 1} s_{l + 2} \dots s_r $ ). You have to construct a binary string $ w $ of length $ n $ which is similar to all of the following strings: $ s[1..n] $ , $ s[2..n+1] $ , $ s[3..n+2] $ , ..., $ s[n..2n-1] $ .

The first line contains a single integer $ t $ ( $ 1 \le t \le 1000 $ ) — the number of test cases. The first line of each test case contains a single integer $ n $ ( $ 1 \le n \le 50 $ ). The second line of each test case contains the binary string $ s $ of length $ 2n - 1 $ . Each character $ s_i $ is either 0 or 1.

For each test case, print the corresponding binary string $ w $ of length $ n $ . If there are multiple such strings — print any of them. It can be shown that at least one string $ w $ meeting the constraints always exists.

4
1
1
3
00000
4
1110000
2
101

1
000
1010
00

The explanation of the sample case (equal characters in equal positions are bold): The first test case: - $ \mathbf{1} $ is similar to $ s[1..1] = \mathbf{1} $ . The second test case: - $ \mathbf{000} $ is similar to $ s[1..3] = \mathbf{000} $ ; - $ \mathbf{000} $ is similar to $ s[2..4] = \mathbf{000} $ ; - $ \mathbf{000} $ is similar to $ s[3..5] = \mathbf{000} $ . The third test case: - $ \mathbf{1}0\mathbf{10} $ is similar to $ s[1..4] = \mathbf{1}1\mathbf{10} $ ; - $ \mathbf{1}01\mathbf{0} $ is similar to $ s[2..5] = \mathbf{1}10\mathbf{0} $ ; - $ \mathbf{10}1\mathbf{0} $ is similar to $ s[3..6] = \mathbf{10}0\mathbf{0} $ ; - $ 1\mathbf{0}1\mathbf{0} $ is similar to $ s[4..7] = 0\mathbf{0}0\mathbf{0} $ . The fourth test case: - $ 0\mathbf{0} $ is similar to $ s[1..2] = 1\mathbf{0} $ ; - $ \mathbf{0}0 $ is similar to $ s[2..3] = \mathbf{0}1 $ .