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