input: Write a function to perform quicksort on a list.
  assert quickSort([3, 2, 5, 7, 6]) == [2, 3, 5, 6, 7]
  assert quickSort([10, 7, 8, 9, 1, 5]) == [1, 5, 7, 8, 9, 10]
  assert quickSort([4, 3, 2, 1]) == [1, 2, 3, 4]
output: def quickSort(arr):
      if len(arr) <= 1:
          return arr
      pivot = arr[len(arr) // 2]
      left = [x for x in arr if x < pivot]
      middle = [x for x in arr if x == pivot]
      right = [x for x in arr if x > pivot]
      return quickSort(left) + middle + quickSort(right)