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 set. If there is a tie for the most frequent value, 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([4, 1, 4, 2, 4, 3])==4
  assert find_mode([3, 5, 7, 3, 7, 5, 3])==3
  assert find_mode([1, 1, 1, 2, 2, 2])==1
  assert find_mode([])==None
output: def find_mode(values):
      if not values:
          return None
      count = {}
      for value in values:
          if value in count:
              count[value] += 1
          else:
              count[value] = 1
      mode = max(count, key=count.get)
      return mode