You have been given a string ‘S’. You need to sort ‘S’ in increasing order based on the frequency of characters. If two characters have the same frequency then the character with a lesser ASCII value must occur first. Return the sorted string as mentioned above.

Example :

Let S= “abAb”. Here the character ‘a’ and ‘A’ with frequency 1 and character ‘b’ with frequency ‘2’. Since ‘a’ and ‘A’ have the same frequency the one with a lower ASCII value will occur first. ‘A’ has a lower ASCII value. After ‘A’, ‘a’ will come and finally ‘b’ as it has the highest frequency. Therefore the sorted string is “Aabb”. 

Note :

1. Strings consist of both lowercase and uppercase alphabet characters.

Input Format :

The first line contains a single integer ‘T’ representing the number of test cases.

The first line of each test case contains a single integer ‘N’ denoting the length of the string ‘S’.

The next line of the test case contains the string ‘S’.

Output Format :

For each test case print the sorted string. 

Note :

You do not need to print anything; it has already been taken care of. Just implement the function.

Constraints :

1 <= T <= 10
1 <= N <= 1000

Time Limit: 1sec