B'378QAQ has a string s of length n . Define the core of a string as the substring ^ dagger with maximum lexicographic ^ ddagger order. For example, the core of " mathtt{bazoka} " is " mathtt{zoka} ", and the core of " mathtt{aaa} " is " mathtt{aaa} ". 378QAQ wants to rearrange the string s so that the core is lexicographically minimum. Find the lexicographically minimum possible core over all rearrangements of s . ^ dagger A substring of string s is a continuous segment of letters from s . For example, " mathtt{defor} ", " mathtt{code} " and " mathtt{o} " are all substrings of " mathtt{codeforces} " while " mathtt{codes} " and " mathtt{aaa} " are not. ^ ddagger A string p is lexicographically smaller than a string q if and only if one of the following holds: For example, " mathtt{code} " and " mathtt{coda} " are both lexicographically smaller than " mathtt{codeforces} " while " mathtt{codeforceston} " and " mathtt{z} " are not. Each test contains multiple test cases. The first line contains the number of test cases t ( 1 <= q t <= q 10^5 ). The description of the test cases follows. The first line of each test case contains a single integer n ( 1 <= q n <= q 10^6 ) -- the length of string s . The next line of each test case contains the string s of length n . The string s consists of lowercase English letters. It is guaranteed that the sum of n over all test cases does not exceed 10^6 . For each test case, output the lexicographically minimum possible core over all rearrangements of s . In the first test case, all possible rearrangements and their corresponding cores are as follows: So the core with the minimum lexicographic order in all rearrangement plans is " mathtt{qaq} ". '...