input: Write a function to return the maximum number in a list.
  
  def find_max(nums):
      """Returns the maximum number in the list."""
      if not nums:
          return None
      max_val = nums[0]
      for num in nums:
          if num > max_val:
              max_val = num
      return max_val
  
  assert find_max([1, 15, 20, 6]) == 20
  assert find_max([4, 6, 2, 9, 1]) == 9
  assert find_max([-2, -5, -1, -3]) == -1
  assert find_max([7]) == 7
  assert find_max([]) == None
output: def find_max(nums):
      """Returns the maximum number in the list."""
      if not nums:
          return None
      max_val = nums[0]
      for num in nums:
          if num > max_val:
              max_val = num
      return max_val