input: Write a python function to sort an array using the quicksort algorithm.
  assert sort_array([3, 6, 8, 10, 1, 2, 1]) == [1, 1, 2, 3, 6, 8, 10]
  assert sort_array([1, 4, 3, 9, 6, 7, 2]) == [1, 2, 3, 4, 6, 7, 9]
  assert sort_array([10, 7, 8, 9, 1, 5]) == [1, 5, 7, 8, 9, 10]
output: def sort_array(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 sort_array(left) + middle + sort_array(right)