input: Write a python function to find the maximum value in a list.
  assert find_max([1, 5, 8, 3, 10]) == 10
  assert find_max([10, 20, 45, 67, 84, 93]) == 93
  assert find_max([5, 7, 9, 8, 6, 4, 3]) == 9
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