input: Write a python function to find the maximum value in a list.
  assert find_max([1, 9, -3, 20, 11]) == 20
  assert find_max([-1, -9, -3, -20, -11]) == -1
  assert find_max([0, 0, 0, 0, 0]) == 0
  assert find_max([5]) == 5
output: def find_max(lst):
      if not lst:
          raise ValueError("List is empty")
      max_value = lst[0]
      for num in lst[1:]:
          if num > max_value:
              max_value = num
      return max_value