input: Write a function to implement bubble sort.
  assert bubble_sort([30, 10, 15, 5, 25]) == [5, 10, 15, 25, 30]
  assert bubble_sort([3, 0, 2, 5, -1, 4, 1]) == [-1, 0, 1, 2, 3, 4, 5]
  assert bubble_sort([100, 90, 80, 70, 60, 50]) == [50, 60, 70, 80, 90, 100]
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