input: Write a function to find the maximum value in a list of numbers.
  assert find_max([3, 4, 6, 5, 9, 1, 2])==9
  assert find_max([10, 20, 30, 40, 50])==50
  assert find_max([-5, -1, -10, -3])==-1
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