input: Write a function to find the maximum value in a list of numbers.
  assert find_max([23, 12, 48, 61]) == 61
  assert find_max([5, 3, 9, 1]) == 9
  assert find_max([-10, -20, -30, -5]) == -5
output: def find_max(numbers):
      if not numbers:
          return None
      max_value = numbers[0]
      for num in numbers[1:]:
          if num > max_value:
              max_value = num
      return max_value