counting sort for strings of letters

#User function Template for python3

class Solution:
    #Function to arrange all letters of a string in lexicographical
    #order using Counting Sort.
    def countSort(self,arr):
        def get(x):
            return ord(x)-ord('a')
        n=len(arr)
        #find frequencies of each character
        count=[0 for i in range(26)]
        for i in arr:
            count[get(i)]+=1
        sum=count[0]
        #make cumulative frequency array
        for i in range(1,26):
            temp=count[i]
            count[i]+=sum
            sum+=temp
        #right rotate array by 1 step
        for i in range(25,-1,-1):
            count[i]=count[i-1]
        count[0]=0
        #resultant array where sorted text exists
        output=[None]*n
        for char in arr:
            index=count[get(char)]
            count[get(char)]+=1
            output[index]=char
        return "".join(output)


        # code here

#{
#  Driver Code Starts
#Initial Template for Python 3

import atexit
import io
import sys

_INPUT_LINES = sys.stdin.read().splitlines()
input = iter(_INPUT_LINES).__next__
_OUTPUT_BUFFER = io.StringIO()
sys.stdout = _OUTPUT_BUFFER

@atexit.register

def write():
    sys.__stdout__.write(_OUTPUT_BUFFER.getvalue())

if __name__ == '__main__':
    test_cases = int(input())
    for cases in range(test_cases) :
        n = int(input())
        arr = str(input())
        ob = Solution()
        print(ob.countSort(arr))

# } Driver Code Ends