How do you validate if a given Tree is a valid BST?

0-2 years experience

1. 1Given a simple binary tree node class, how would you write a function to check if the tree is a valid BST? Walk me through your approach.
2. 2If the tree contains duplicate values, how would you decide whether they should be considered valid in a BST validation?
3. 3What would happen if you performed an in‑order traversal but forgot to handle the case where the left subtree is empty?

2-5 years experience

1. 1Our service receives a serialized tree from a client and we need to verify it's a BST before processing. The current recursive min/max check sometimes crashes on deep trees. How would you debug and improve it?
2. 2We switched from a recursive to an iterative in‑order traversal for BST validation to avoid stack overflow. What trade‑offs should we consider regarding time, space, and code complexity?
3. 3During a recent release, a tree with values [5,5] was incorrectly marked as valid. Explain why the existing validation fails and how you'd fix it.

5-8 years experience

1. 1Our database stores millions of BSTs and runs a nightly job to validate each one. Design an approach that can validate trees in parallel while keeping memory usage low.
2. 2We need to add BST validation to a distributed microservice that receives partial tree fragments from different nodes. How would you ensure the overall structure is a valid BST without sending the entire tree over the network?
3. 3Explain how you would modify the validation algorithm to also check balance factors for a self‑balancing BST like an AVL tree efficiently.

8+ years experience

1. 1Our legacy codebase has many custom tree implementations across services. We want to standardize BST validation organization‑wide. What architectural strategy would you propose to enforce a consistent, testable validation contract while minimizing impact on existing services?
2. 2We are migrating from an on‑prem monolith to a cloud‑native platform, moving tree‑based indexes to a distributed store. How would you design a validation pipeline that can verify BST properties at scale and integrate with CI/CD?
3. 3Multiple teams need to extend the tree node with extra metadata (timestamps, permissions). How would you design the validation API to stay robust and type‑safe across languages and future extensions?