How linked-list questions give you the chain

Each link is a small Link object with a value and a next reference; the last link's next is None. Pages take the head link. To test, build a chain from a Python list with a short helper and read it back with another, so test cases stay readable.

The same two helpers sit behind every linked-list page, so the tests use plain lists.

eq=False keeps identity comparison, which loop checks need.

On a whiteboard, draw boxes and arrows first; it is faster than reasoning in code.

from dataclasses import dataclass


@dataclass(eq=False)
class Link:
    val: int
    next: "Link | None" = None  # None at the tail


def build(vals: list[int]) -> Link | None:
    head: Link | None = None
    for v in reversed(vals):
        head = Link(v, head)
    return head


def to_vals(head: Link | None) -> list[int]:
    out: list[int] = []
    while head is not None:
        out.append(head.val)
        head = head.next
    return out


print(to_vals(build([4, 7, 1])))

Common mistakes

• Printing a looping chain, which never ends.
• Comparing links with == when you mean the same object (is).

Offer to write the build helper first; it makes your own tests quick.

Follow-up questions

Think of your answer first, then open the hint.

1. Why not use a Python list in the answer?

   Show hint

   The question is about pointer moves with O(1) extra memory; a list copy changes the cost.

2. How would you print a chain that may loop?

   Show hint

   Stop after a step limit or track visited links in a set.