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([-1, -5, -2, -10]) == -1
  assert find_max([3, 7, 3, 3, 7]) == 7
  assert find_max([]) == None
output: def find_max(nums):
      if not nums:
          return None
      max_val = nums[0]
      for num in nums:
          if num > max_val:
              max_val = num
      return max_val