input: Write a python function to find the maximum value in a list.
  assert find_max([1, 8, 2, 4, 10]) == 10
  assert find_max([5, 3, 9, 0, -1]) == 9
  assert find_max([1, 1, 1, 1, 1]) == 1
output: def find_max(lst):
      if not lst:
          return None
      max_value = lst[0]
      for num in lst:
          if num > max_value:
              max_value = num
      return max_value