C. Phoenix and Distributiontime limit per test2 secondsmemory limit per test256 megabytesinputstandard inputoutputstandard outputPhoenix has a string s consisting of lowercase Latin letters. He wants to distribute all the letters of his string into k non-empty strings a_1, a_2, \dots, a_k such that every letter of s goes to exactly one of the strings a_i. The strings a_i do not need to be substrings of s. Phoenix can distribute letters of s and rearrange the letters within each string a_i however he wants.For example, if s =  baba and k=2, Phoenix may distribute the letters of his string in many ways, such as:   ba and ba  a and abb  ab and ab  aa and bb But these ways are invalid:   baa and ba  b and ba  baba and empty string (a_i should be non-empty) Phoenix wants to distribute the letters of his string s into k strings a_1, a_2, \dots, a_k to minimize the lexicographically maximum string among them, i. e. minimize max(a_1, a_2, \dots, a_k). Help him find the optimal distribution and print the minimal possible value of max(a_1, a_2, \dots, a_k).String x is lexicographically less than string y if either x is a prefix of y and x \ne y, or there exists an index i (1 \le i \le min(|x|, |y|)) such that x_i < y_i and for every j (1 \le j < i) x_j = y_j. Here |x| denotes the length of the string x.InputThe input consists of multiple test cases. The first line contains an integer t (1 \le t \le 1000) — the number of test cases. Each test case consists of two lines.The first line of each test case consists of two integers n and k (1 \le k \le n \le 10^5) — the length of string s and the number of non-empty strings, into which Phoenix wants to distribute letters of s, respectively.The second line of each test case contains a string s of length n consisting only of lowercase Latin letters.It is guaranteed that the sum of n over all test cases is \le 10^5.OutputPrint t answers — one per test case. The i-th answer should be the minimal possible value of max(a_1, a_2, \dots, a_k) in the i-th test case.ExampleInput
6
4 2
baba
5 2
baacb
5 3
baacb
5 3
aaaaa
6 4
aaxxzz
7 1
phoenix
Output
ab
abbc
b
aa
x
ehinopx
NoteIn the first test case, one optimal solution is to distribute baba into ab and ab. In the second test case, one optimal solution is to distribute baacb into abbc and a.In the third test case, one optimal solution is to distribute baacb into ac, ab, and b.In the fourth test case, one optimal solution is to distribute aaaaa into aa, aa, and a.In the fifth test case, one optimal solution is to distribute aaxxzz into az, az, x, and x.In the sixth test case, one optimal solution is to distribute phoenix into ehinopx.
6
4 2
baba
5 2
baacb
5 3
baacb
5 3
aaaaa
6 4
aaxxzz
7 1
phoenix

ab abbc b aa x ehinopx 
['constructive algorithms', 'greedy', 'sortings', 'strings', '*1600']