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