input: Write a python function to find the maximum value in a list.
  assert find_max([1, 2, 3, 4, 5]) == 5
  assert find_max([10, 20, 30, 40, 50]) == 50
  assert find_max([-10, -20, -30, -40, -50]) == -10
output: def find_max(lst):
      if not lst:
          return None
      max_val = lst[0]
      for num in lst[1:]:
          if num > max_val:
              max_val = num
      return max_val