input: Write a function to find the common elements in two lists and return them as a list. If there are no common elements, return an empty list. If either list is empty, return an empty list.
  def find_common_elements(list1, list2):
      common_elements = []
      for i in list1:
          if i in list2:
              common_elements.append(i)
      return common_elements
  
  assert find_common_elements([3, 9, 15, 21, 27], [21, 7, 13, 29, 31])==[21]
  assert find_common_elements([1, 2, 3, 4], [5, 6, 7, 8])==[]
  assert find_common_elements([10, 20, 30], [30, 10])==[10, 30]
  assert find_common_elements([], [1, 2, 3])==[]
  assert find_common_elements([1, 2, 3], [])==[]
  assert find_common_elements([], [])==[]
output: def find_common_elements(list1, list2):
      if not list1 or not list2:
          return []
      return [i for i in list1 if i in list2]