input: Write a python function to implement bubble sort.
  assert bubble_sort([4,7,2,9,1])==[1,2,4,7,9]
  assert bubble_sort([10,5,3,8,2])==[2,3,5,8,10]
  assert bubble_sort([1,2,3,4,5])==[1,2,3,4,5]
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