input: Write a function to sort a given array in ascending order using bubble sort. If the given array is empty, return the array as is.
  
  def bubble_sort(arr):
      n = len(arr)
      for i in range(n-1):
          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
  
  arr1 = [3,7,4,1]
  arr2 = [9,1,5,3,2]
  arr3 = []
  
  assert bubble_sort(arr1) == [1, 3, 4, 7]
  assert bubble_sort(arr2) == [1, 2, 3, 5, 9]
  assert bubble_sort(arr3) == []
output: def bubble_sort(arr):
      n = len(arr)
      for i in range(n-1):
          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