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([64, 34, 25, 12, 22, 11, 90])==[11,12,22,25,34,64,90]
  assert bubble_sort([1,2,3,4,5])==[1,2,3,4,5]
  assert bubble_sort([5,4,3,2,1])==[1,2,3,4,5]
  assert bubble_sort([])==[]
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]