input: Write a function to sort a given array in ascending order using bubble sort.
  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]
  
  assert bubble_sort([3,7,4,1]) == [1, 3, 4, 7]
  assert bubble_sort([5,2,9,1,5,6]) == [1, 2, 5, 5, 6, 9]
  assert bubble_sort([]) == []
  assert bubble_sort([1]) == [1]
  assert bubble_sort([7,7,7,7]) == [7,7,7,7]
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]