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Xai·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say
Apr 2026

Summary

Interviewed for a software engineer role at xAI and got a recursive data structure problem. Pretty classic but the edge cases kept me on my toes longer than I expected.

Questions Asked (1)

Q1

Write a function that takes an arbitrarily nested Python data structure (which can contain lists, dicts, and tuples) with integer leaf values, and returns a flat list of all integers in the order they are encountered.

Algorithms & Data StructuresTechnical Trade-offs
Author's notes

I went straight for recursion, which was the right call, but I fumbled the dict case initially.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the problem: confirm that the structure can be arbitrarily nested and that the order is depth-first, left-to-right. Then propose a recursive solution that traverses each container type, handling lists/tuples by iterating and dicts by iterating over values (or items if keys are needed), and collecting integers into a result list. Discuss iterative alternatives and trade-offs briefly.

Pro tip: Mention that you would use a helper function with an accumulator to avoid repeated list concatenation, and that you'd test with edge cases like empty containers, deeply nested structures, and mixed types. This shows awareness of performance and robustness.

1. Clarify requirements and assumptions

Confirm the definition of 'order' (depth-first, left-to-right), whether dict keys should be included, and if the structure can contain other types. Ask about input size and recursion depth limits.

2. Choose traversal strategy

Decide between recursive and iterative approaches. Recursion is simpler and more readable; iteration with an explicit stack avoids recursion limits. Mention that both are valid and discuss trade-offs.

3. Implement the traversal

Write a function that checks if the current element is an integer (leaf) and appends it to the result; if it's a list or tuple, iterate over its elements; if it's a dict, iterate over its values (or items if keys are needed). Recurse or push onto stack accordingly.

4. Test with edge cases

Test with empty containers, nested empty containers, deeply nested structures, and mixed types. Verify order matches depth-first left-to-right. Consider performance for large inputs.

5. Discuss trade-offs and optimizations

Compare recursive vs iterative in terms of readability, stack overflow risk, and performance. Mention using a generator for lazy evaluation if memory is a concern, or an accumulator to avoid list concatenation overhead.

Key Points to Mention

  • Depth-first traversal order (pre-order) and how it applies to nested structures.
  • Handling different container types: lists, tuples, and dicts (values vs items).
  • Recursive vs iterative implementation trade-offs (readability, stack depth, performance).
  • Using an accumulator or generator to avoid O(n^2) list concatenation.
  • Edge cases: empty containers, deeply nested structures, non-integer leaves.
  • Time and space complexity: O(n) time where n is total number of elements, O(d) space for recursion depth d.

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