Linked-list boundaries and rewiring

Opening your lesson

Linked-list changes depend on preserving access to the suffix before overwriting a link. Draw the current node, its successor, and the first untouched node. Save the pointer you still need, then change links in an order that can't lose it. A dummy predecessor lets the first group use the same rewiring logic as every later group.

The exercises progress through sorted deduplication, rotation, adjacent swaps, stable partitioning, and complete-group reversal. Empty lists and one-node lists make useful first traces because they reveal unsafe dereferences. For rotation, a temporary ring must be cut back into a chain. For partitioning, each group needs its own tail. For group reversal, confirm k nodes exist before changing anything so a short final suffix remains in its original order.

before -> A -> B -> C -> D
first = A; second = B
first.next = C
second.next = A
before.next = B
before = A
result: before-old -> B -> A -> C -> D

The first assignment preserves the untouched suffix beginning at C by attaching it to A. Then B points back to A, and the predecessor points forward to B. The pair is now reversed while the route remains connected. Advancing the predecessor to A positions it immediately before the next pair, C and D. A dummy node provides that predecessor for the first pair, where there is no earlier real node. Returning dummy.next is necessary because the original head A is no longer first. The same saved-boundary discipline applies when you reverse larger complete groups instead of pairs.

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