input: Write a function to combine two lists by interleaving their elements.
  assert combine_list([1, 2, 3], [4, 5, 6]) == [1, 4, 2, 5, 3, 6]
  assert combine_list([1, 2], [3, 4, 5, 6]) == [1, 3, 2, 4, 5, 6]
  assert combine_list([], [1, 2, 3]) == [1, 2, 3]
output: def combine_list(list1, list2):
      result = []
      for a, b in zip(list1, list2):
          result.append(a)
          result.append(b)
      result.extend(list1[len(result)//2:])
      result.extend(list2[len(result)//2:])
      return result