input: Write a python function to interleave two lists.
  assert concat_lists([1, 2, 3], [4, 5, 6]) == [1, 4, 2, 5, 3, 6]
  assert concat_lists(['a', 'b', 'c'], ['d', 'e', 'f']) == ['a', 'd', 'b', 'e', 'c', 'f']
  assert concat_lists([10, 20], [30, 40, 50]) == [10, 30, 20, 40]
  assert concat_lists([7, 8, 9], [1, 2]) == [7, 1, 8, 2]
output: def concat_lists(list1, list2):
      result = []
      length = max(len(list1), len(list2))
      for i in range(length):
          if i < len(list1):
              result.append(list1[i])
          if i < len(list2):
              result.append(list2[i])
      return result