input: Write a function to find the most common elements in an array of integers. If there are multiple elements with the same highest frequency, return all of them. The input array is guaranteed to have at least one element.
  assert most_common([3, 4, 1, 2, 5, 3, 3])==[3]
  assert most_common([1, 1, 2, 2, 2, 3, 3, 3])==[2, 3]
  assert most_common([7])==[7]
  assert most_common([5, 5, 5, 4, 4, 4, 5])==[5]
  assert most_common([-1, -1, 0, 0, -1])==[-1]
output: def most_common(arr):
      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]