input: Write a function to find the most common elements in an array of integers. Return the elements that have the highest frequency. If there are multiple elements with the same highest frequency, return all of them.
  assert most_common([3, 4, 1, 2, 5, 3, 3])==[3]
  assert most_common([4, 4, 1, 2, 5, 2, 2, 4])==[2, 4]
  assert most_common([1, 2, 3, 4, 5])==[1, 2, 3, 4, 5]
  assert most_common([10, 20, 30, 40, 50, 10, 20])==[10, 20]
  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]