input: Write a function to implement bubble sort.
  assert bubble_sort([12, 11, 13, 5, 6]) == [5, 6, 11, 12, 13]
  assert bubble_sort([3, 2, 1]) == [1, 2, 3]
  assert bubble_sort([5, 5, 5, 5]) == [5, 5, 5, 5]
  assert bubble_sort([10, -1, 2, 5, 0]) == [-1, 0, 2, 5, 10]
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