input: Write a python function to find the maximum element in a list.
  assert find_max([1, 2, 3, 4, 5]) == 5
  assert find_max([10, 20, 5, 7, 3]) == 20
  assert find_max([-1, -2, -3, -4, -5]) == -1
output: def find_max(lst):
      if not lst:
          return None
      max_element = lst[0]
      for element in lst[1:]:
          if element > max_element:
              max_element = element
      return max_element