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Meta·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
Apr 2026

Summary

Meta data engineer interview with a tree validation problem. Pretty standard technical screen, nothing too wild, but it's one of those questions that looks easy until you start coding it up.

Questions Asked (1)

Q1

Given a binary tree, write code to determine whether it is a valid binary search tree.

Algorithms & Data Structures
Author's notes

I went straight for the naive approach, checking left child less than root and right child greater than root at each node.

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AI HintsAI Generated

Suggested Approach

Clarify the definition of a BST (strict ordering, no duplicates) and then propose a recursive solution that passes down a valid range (min, max) for each node. Alternatively, use an in-order traversal to check if the sequence is strictly increasing. Discuss time and space complexity, and handle edge cases like empty tree or single node.

Pro tip: Mention that a common mistake is only checking immediate children; instead, emphasize that each node must satisfy constraints from all ancestors. Also, consider iterative in-order traversal to achieve O(1) space if recursion stack is a concern.

1. Clarify requirements

Ask whether duplicates are allowed and if the BST definition is strict (left < root < right). Confirm input format and expected output.

2. Choose approach

Decide between recursive range-checking or iterative in-order traversal. Explain trade-offs in time/space complexity.

3. Implement solution

Write clean code with helper functions. For range-checking, pass min and max bounds; for in-order, track previous node value.

4. Test with examples

Walk through edge cases: empty tree, single node, invalid BST where a node violates ancestor constraints, and tree with duplicates (if allowed).

5. Analyze complexity

State time complexity O(n) and space complexity O(h) for recursion or O(1) for iterative with Morris traversal. Discuss optimizations.

Key Points to Mention

  • Definition of BST: left subtree values < node value < right subtree values, with strict inequality unless duplicates specified.
  • Common pitfall: only comparing node with immediate children, missing violations from ancestors.
  • Recursive range-checking: pass down (min, max) bounds, initially (-inf, +inf).
  • In-order traversal: yields sorted sequence if valid BST; check if strictly increasing.
  • Time complexity O(n) and space complexity O(h) for recursion, O(1) for iterative with Morris traversal.
  • Handling duplicates: if allowed, adjust comparison to <= or >=, but clarify with interviewer.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.