input: Write a function to sort a given array in ascending order using bubble sort.
  assert bubble_sort([3, 7, 4, 1])==[1, 3, 4, 7]
  assert bubble_sort([5, 2, 9, 1])==[1, 2, 5, 9]
  assert bubble_sort([10, 8, 6, 4])==[4, 6, 8, 10]
  assert bubble_sort([1, 2, 3, 4])==[1, 2, 3, 4]
  assert bubble_sort([])==[]
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