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Xai·Machine Learning Engineer·Technical Phone Screen·Senior

Senior
Jun 2026

Summary

xAI ML Engineer interview with a pretty gnarly algorithmic puzzle about fault detection in a node graph. No behavioral stuff, just pure problem-solving under pressure. Left feeling like I'd gotten maybe 70% of the way there.

Questions Asked (1)

Q1

You have N nodes, each secretly either good or bad. You can call a test function on any subset of 2+ nodes that returns true only if every node in the subset is good. Tests within the same round must use disjoint sets of nodes. Design an algorithm to identify all bad nodes, and analyze its round and call complexity.

Algorithms & Data StructuresTechnical Trade-offsSystem Design
Author's notes

This wrecked me a little.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the problem constraints and edge cases, then propose a divide-and-conquer strategy that uses group testing to identify bad nodes efficiently. Analyze the round and call complexity, and discuss trade-offs between the two.

Pro tip: Mention that this is a variant of group testing and that adaptive strategies can reduce calls, but non-adaptive may be needed for round efficiency. Also, note that identifying all bad nodes is equivalent to finding the complement of the set of good nodes.

1. Clarify the problem

Confirm that tests return true only if all nodes in the subset are good, and that tests in the same round must be disjoint. Ask about constraints on N and whether the number of bad nodes is known.

2. Design a baseline algorithm

Propose a simple approach: test all nodes in pairs or small groups to identify good nodes, then deduce bad nodes. Analyze its round and call complexity.

3. Optimize with divide-and-conquer

Use a binary splitting strategy: recursively split the set of nodes into subsets, test each subset, and if a test returns false, recurse on that subset to find bad nodes. Ensure tests within a round are disjoint.

4. Analyze complexity

Derive the number of rounds and calls in terms of N and the number of bad nodes. Compare with the baseline and discuss trade-offs.

5. Discuss extensions and practical considerations

Consider noisy tests, unknown number of bad nodes, and how the algorithm scales. Mention potential applications in ML (e.g., data cleaning, ensemble methods).

Key Points to Mention

  • Group testing and its connection to this problem
  • Divide-and-conquer approach for identifying bad nodes
  • Round complexity: how many parallel rounds are needed
  • Call complexity: total number of tests performed
  • Trade-offs between adaptive and non-adaptive strategies
  • Edge cases: all nodes good, all nodes bad, single bad node

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