input: Write a function to return the maximum number in a list. If the list is empty, return None.
  
  def find_max(nums):
      '''Returns the maximum number in the list. If the list is empty, returns None.'''
      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([3, 5, 10, 1, 2])==10
  assert find_max([4, 3, 1, 2])==4
  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