input: Write a python function to find the maximum value in a list.
  assert find_max([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]) == 10
  assert find_max([-1, -2, -3, -4, -5]) == -1
  assert find_max([10, 20, 30, 5, 15]) == 30
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