LeetCode Card 270 — Binary Search Trees / Check
Pattern signal
This pattern usually appears when the prompt involves ordered tree properties, rank queries, or pruning based on node value. Validate BST, Kth Smallest Element in a BST, Lowest Common Ancestor of a BST, and range queries are the usual forms.
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 Binary Search Trees, 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 treating a BST like a generic tree and ignoring the ordering information that should prune work. 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 Binary Search Trees, what is the simplest way to dry-run the result on edge cases, confirm complexity, and inspect whether the invariant actually held?”