input: Write a function to implement the bubble sort algorithm that sorts a given array in ascending order. If the input array is empty, the function should return the empty array as it is.
  assert bubble_sort([3, 7, 4, 1])==[1, 3, 4, 7]
  assert bubble_sort([9, 8, 7, 6, 5])==[5, 6, 7, 8, 9]
  assert bubble_sort([1, 2, 3, 4, 5])==[1, 2, 3, 4, 5]
  assert bubble_sort([])==[]
  assert bubble_sort([4, 3, 2, 1])==[1, 2, 3, 4]
output: def bubble_sort(arr):
      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