input: Write a function to return the maximum number in a list. The function should iterate through the list and keep track of the maximum value found. If the list is empty, it should return None.
  assert find_max([1, 15, 20, 6]) == 20
  assert find_max([4, 8, 2, 10, 5]) == 10
  assert find_max([]) == None
  assert find_max([-1, -5, -3, -2]) == -1
  assert find_max([7]) == 7
output: def find_max(lst):
      if not lst:
          return None
      max_val = lst[0]
      for num in lst:
          if num > max_val:
              max_val = num
      return max_val