input: Write a function to find the mode of the given set of values, where the mode is the value that appears most frequently in the list. If there is a tie for the mode, return the first one that appears.
  assert find_mode([1, 2, 3, 2, 4, 2])==2
  assert find_mode([1, 2, 3, 3, 4, 2, 3])==3
  assert find_mode([1, 2, 3, 4, 5])==1
  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