input: Write a function to find the mode of a given set of values. The mode is the number that appears most frequently in the set. If multiple numbers have the same highest frequency, return the first one encountered.
  
  assert find_mode([1, 2, 3, 2, 4, 2])==2
  assert find_mode([1, 1, 2, 2, 3, 3, 3])==3
  assert find_mode([5])==5
  assert find_mode([10, 10, 20, 30, 20, 20])==20
  assert find_mode([])==None
output: def find_mode(values):
      if not values:
          return None
      frequency = {}
      for num in values:
          if num in frequency:
              frequency[num] += 1
          else:
              frequency[num] = 1
      mode = max(frequency, key=frequency.get)
      return mode