input: Write a function to find the most common elements in an array of integers. The function should return a list of these elements. If the array is empty, the function should return an empty list.
  
  assert most_common([3, 4, 1, 2, 5, 3, 3])==[3]
  assert most_common([1, 2, 3, 4, 5, 5, 5, 4, 3, 2])==[3, 4, 5]
  assert most_common([1, 1, 2, 2, 3, 3, 4, 4])==[1, 2, 3, 4]
  assert most_common([1, 1, 1, 1, 1])==[1]
  assert most_common([])==[]
output: def most_common(arr):
      if not arr:
          return []
      count = {}
      for num in arr:
          if num in count:
              count[num] += 1
          else:
              count[num] = 1
      max_count = max(count.values())
      return [num for num in count if count[num] == max_count]