How do you validate that a binary tree is a BST?
PICTURE THIS: HOW TO EXPLAIN IT
Simple meaning
DFS with a legal (low, high) window: left child must be < node and stay > low, right must be > node and stay < high.
WHY — BST instead of guessing?
Why interviewers care about BST:
question about BST.
trade-offs, and what you would actually do on a DSA project - not buzzwords.
Name the idea, why it exists, then one short example.
End with when you use it and one common pitfall.
STEPS — What happens step by step?
Before you speak the answer, walk the interviewer through these steps:
- 1DFS with a legal
(low, high) window: left child must be < node and stay > low, right must be > node and stay < high.
- 2Time is O(n) and
space is O(h).
- 3Inorder and checking non-decreasing
values also works
- 4watch duplicates if the
tree must be strictly increasing.
- 5Common mistake
What juniors usually get wrong.
- 6Close
When you pick this over the alternative.
EXAMPLE — See it in action
Here's a short line you can speak, broken into clear beats:
Note: Adapt this scaffold to your own project — keep it under 60–90 seconds.
Key takeaway
DFS with a legal (low, high) window: left child must be < node and stay > low, right must be > node and stay < high. Time is O(n) and space is O(h).