Phoenix and Distribution

题意翻译

将给出的字符串分成若干个部分，每部分不重复、不需要连续或者按顺序。 如字符串"abbac"可以被分成"acb"和"ab"。 求长度为$n$的字符串$s$分成$k$个部分，$k$部分中字典序最大的最小字典序。 translate by luogu @$\text{longer\_name}$

题目描述

Phoenix 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 $ &lt; $ y_i $ and for every $ j $ $ (1 \le j < i) $ $ x_j = y_j $ . Here $ |x| $ denotes the length of the string $ x $ .

输入输出格式

输入格式

The 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 $ .

输出格式

Print $ 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.

输入输出样例

输入样例 #1

6
4 2
baba
5 2
baacb
5 3
baacb
5 3
aaaaa
6 4
aaxxzz
7 1
phoenix

输出样例 #1

ab
abbc
b
aa
x
ehinopx

说明

In 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.