input: Write a function that takes a list of integers as input and returns a list of the most common elements. If there are multiple elements with the same highest frequency, it should return all of them. If the input list is empty, return an empty list.
  assert most_common([3, 4, 1, 2, 5, 3, 3])==[3]
  assert most_common([1, 2, 2, 3, 3, 4])==[2, 3]
  assert most_common([5])==[5]
  assert most_common([])==[]
output: def most_common(lst):
      if not lst:
          return []
      freq = {}
      for num in lst:
          if num in freq:
              freq[num] += 1
          else:
              freq[num] = 1
      max_freq = max(freq.values())
      return [num for num in freq if freq[num] == max_freq]