input: Write a function to calculate the average of a list of positive integers and output the result. If the input list is empty, return 0.
  assert average_of_list([1, 5, 6, 7, 8]) == 5.4
  assert average_of_list([10, 20, 30, 40, 50]) == 30.0
  assert average_of_list([5]) == 5.0
  assert average_of_list([]) == 0
  assert average_of_list([9, 9, 9, 9]) == 9.0
output: def average_of_list(lst):
      if not lst:
          return 0
      return sum(lst) / len(lst)