LeetCode Card 225 — Sliding Window / Check
Pattern signal
This pattern usually appears when the prompt involves contiguous substring or subarray optimization with a condition that changes as the window moves. Classic examples include Longest Substring Without Repeating Characters, Minimum Window Substring, and fixed-size sum windows.
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 Sliding Window, 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
The miss is not separating what causes the window to expand from what causes it to shrink. 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 Sliding Window, what is the simplest way to dry-run the result on edge cases, confirm complexity, and inspect whether the invariant actually held?”