input: Write a python function to search for a node with a specific key in a linked list.
  assert search(head, 5).data == 5
  assert search(head, 10) is None
  assert search(head, 3).data == 3
output: def search(head, key):
      current = head
      while current is not None:
          if current.data == key:
              return current
          current = current.next
      return None