input: Write a function to find the mode of the given set of values. The mode is the value that appears most frequently in the list. If there are multiple modes, return any one of them. If the list is empty, return None.
  
  assert find_mode([1, 2, 3, 2, 4, 2])==2
  assert find_mode([1, 3, 3, 2, 4, 2, 3])==3
  assert find_mode([1, 2, 3, 4, 5])==1
  assert find_mode([1, 1, 2, 2, 3, 3, 4, 4]) in [1, 2, 3, 4]
  assert find_mode([])==None
output: def find_mode(values):
      if not values:
          return None
      frequency = {}
      for value in values:
          if value in frequency:
              frequency[value] += 1
          else:
              frequency[value] = 1
      mode = max(frequency, key=frequency.get)
      return mode