input: from typing import List
  
  def bubble_sort(arr: List[int]): 
      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] 
  
      """
      Sorts a given array in ascending order using bubble sort.
      >>> arr = [3, 7, 4, 1]
      >>> bubble_sort(arr)
      >>> arr
      [1, 3, 4, 7]
      """
output:     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]