input: Write a python function to find common elements between two lists.
  assert common_elements([2, 3, 4, 5], [1, 2, 4, 5]) == [2, 4, 5]
  assert common_elements([1, 2, 3], [4, 5, 6]) == []
  assert common_elements([7, 8, 9], [9, 8, 7]) == [7, 8, 9]
output: def common_elements(list1, list2):
      return list(set(list1) & set(list2))