input: def bubble_sort(arr):
      """ Bubble sort an array in ascending order.
      >>> bubble_sort([3, 7, 4, 1])
      [1, 3, 4, 7]
      >>> bubble_sort([9, 8, 7, 6])
      [6, 7, 8, 9]
      """
output:     n = len(arr)
      for i in range(n):
          for j in range(0, n-i-1):
              if arr[j] > arr[j+1]:
                  arr[j], arr[j+1] = arr[j+1], arr[j]
      return arr