Remove Cycle
After slow and fast meet, walk from the head and the meeting point to the entry, then once round the loop to cut the arrow before it.
Decision · step 2 of 10List Pointer Puzzles: Cut a loop back to the third node
Slow to 2, fast to 3. Not the same node yet.
What you will see
The arrow that closes the loop is cut.
Cost
| Best | O(n) |
|---|---|
| Average | O(n) |
| Worst | O(n) |
| Space | O(1) |
How you work with it here
play it through, step one change at a time, scrub to any step, run it on your own input, predict what happens next, try operations in any order.
Screen readers: Each node announces its value and what it points to; each step announces the pointer change ("node 3 now points to node 5").
Reduced motion: Pointer arrows redraw in place with a brief emphasis instead of animating the detach and reattach.
Before this
Taught by the same lesson
List Pointer Puzzles covers these too, in the same run.