LeetCode Card 255 — Fast and Slow Pointers / Check
Pattern signal
This pattern usually appears when the prompt involves cycle detection, midpoint discovery, or relative-speed traversal. Linked List Cycle, Palindrome Linked List, and finding list middles are canonical examples.
What Check means here
In BASIC, the Check step is where you dry-run the result on edge cases, confirm complexity, and inspect whether the invariant actually held. For Fast and Slow Pointers, that matters because the pattern only becomes useful once the candidate is explicit about what is being tracked, reduced, or preserved.
Interview move
A strong move is to review and stress-test before you hand the answer over. In this pattern family, say out loud what representation makes the problem easier: the map entry, the pointer invariant, the recursion contract, the queue contents, or the DP state. That keeps the implementation attached to a reason.
Common miss
Candidates know the trick but cannot explain why the pointer speeds produce the desired meeting or split. BASIC reduces that risk because the stage sequence forces you to earn the implementation instead of jumping straight into it.
BASIC prompt
“Given that this looks like Fast and Slow Pointers, what is the simplest way to dry-run the result on edge cases, confirm complexity, and inspect whether the invariant actually held?”