How do you find the Lowest Common Ancestor (LCA) of two nodes in a BST?

0-2 years experience

1. 1Given a BST and two node values, how would you write a function to return their lowest common ancestor?
2. 2If the BST is unbalanced and one of the nodes is the root, what will your LCA algorithm return?
3. 3How would you modify the algorithm to return null when either of the target nodes does not exist in the tree?

2-5 years experience

1. 1Our service stores user permissions in a BST and we need to find the deepest shared permission node for two users; how would you adapt the LCA algorithm, and what edge cases would you watch for?
2. 2During a recent deployment, the LCA function started returning null for some node pairs; how would you debug it given the BST may contain duplicate values?
3. 3If the tree is being modified concurrently, what considerations would you add to ensure the LCA computation remains correct?

5-8 years experience

1. 1We plan to cache LCA results for frequent queries in a large‑scale recommendation engine; discuss the tradeoffs of caching versus recomputing on each request.
2. 2When the BST is stored on disk and traversed via I/O, how would you redesign the LCA algorithm to minimize disk seeks?
3. 3If the BST grows to billions of nodes and we need sub‑millisecond LCA latency, what indexing or auxiliary data structures would you consider?

8+ years experience

1. 1Our legacy monolith uses an in‑memory BST for hierarchical data, but we are moving to a microservices architecture with a distributed graph store; how would you migrate LCA functionality while preserving API contracts?
2. 2Across teams, some services use AVL trees while others use Red‑Black trees; how would you create a unified LCA service that abstracts away the underlying tree implementation?
3. 3What long‑term maintenance concerns arise from exposing LCA as a public API, and how would you version it to handle future changes in tree structure?