input: Write a function to find the mode of a given set of values. The mode is the value that appears most frequently in the list. If there are multiple values with the same highest frequency, 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, 5, 5, 5])==5
  assert find_mode([1, 2, 3, 4, 5])==1
  assert find_mode([7])==7
  assert find_mode([])==None
output: def find_mode(values):
      if not values:
          return None
      freq = {}
      for value in values:
          if value in freq:
              freq[value] += 1
          else:
              freq[value] = 1
      mode = max(freq, key=freq.get)
      return mode