Problem context and objectives
Mission briefing
Portal network mechanic · Travel portals are connected as a one-way chain
Repair the links without losing the unexplored route: The Vanishing Link.
Overwrite one next pointer too early and the rest of the expedition becomes unreachable.
How you win
- 1Recognize when Predecessor rewiring matches the clues
- 2Keep this true after every move: lead remains n links ahead of the predecessor pointer
- 3Reach the result within O(n)
Rules and pressure
- Target cost: O(n)
- State rule: lead remains n links ahead of the predecessor pointer
Lesson 1 of 3
Send lead n + 1 links ahead from dummy. Then march both pointers together; when lead reaches null, the follower is immediately before the target.
Your call · When removing the second node from the end of [1,2,3,4,5], where does predecessor stop?