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Openai·Software Engineer·Technical Phone Screen·Senior

Senior
Apr 2026

Summary

Code reading round at OpenAI for a SWE role. You get a 400-line PyTorch file and have to work through three parts plus a bonus, with complexity questions sprinkled throughout on the existing code, not just whatever you write.

Questions Asked (2)

Q1

What is the time and space complexity of a matrix multiplication operation A @ B in this codebase?

Algorithms & Data StructuresTechnical Trade-offs
Author's notes

This sounds like a warmup but it isn't.

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AI HintsAI Generated

Suggested Approach

First, clarify the dimensions of matrices A and B and the specific multiplication algorithm used in the codebase. Then, derive the time and space complexity based on the algorithm, considering any optimizations or library calls. Finally, discuss trade-offs and potential improvements.

Pro tip: Mention that in practice, libraries like BLAS or cuBLAS use blocked or Strassen-like algorithms, so the theoretical O(n^3) may not reflect actual performance. Also, highlight that space complexity often includes the output matrix, which is O(n^2).

1. Clarify matrix dimensions and multiplication method

Ask or state the dimensions of A (m x n) and B (n x p) and whether the code uses a naive triple-loop, a library call (e.g., numpy.dot), or a custom algorithm.

2. Derive time complexity

For standard matrix multiplication, time complexity is O(m * n * p). If square matrices (n x n), it's O(n^3). Mention that optimized algorithms like Strassen reduce this to O(n^2.807).

3. Derive space complexity

The output matrix C is m x p, so space complexity is O(m * p). If in-place or using additional buffers, account for that. For square matrices, it's O(n^2).

4. Consider practical optimizations and trade-offs

Discuss how cache blocking, parallelization, or GPU acceleration affect actual performance and may change the effective complexity. Mention that space-time trade-offs exist (e.g., using extra memory to reduce time).

5. Summarize and relate to the codebase

Conclude with the specific complexity in the given codebase, referencing any library or implementation details you observed or would look for.

Key Points to Mention

  • Standard matrix multiplication time complexity: O(m * n * p) or O(n^3) for square matrices.
  • Space complexity: O(m * p) for the output matrix, often O(n^2) for square matrices.
  • Optimized algorithms like Strassen (O(n^2.807)) or Coppersmith-Winograd (O(n^2.376)) exist but are rarely used in practice due to overhead.
  • Libraries like BLAS, LAPACK, or deep learning frameworks often use blocked algorithms and hardware acceleration, affecting real-world performance.
  • Trade-offs: time vs. space, and the impact of cache efficiency and parallelism.
  • The importance of clarifying dimensions and implementation details before answering.

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

Q2

Given this unfamiliar 400-line PyTorch file, walk us through the data flow and explain what the code is doing.

Technical Trade-offsAdaptability & Ambiguity
Author's notes

The file is messy on purpose.

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AI HintsAI Generated

Suggested Approach

Start by skimming the file to identify the main components: imports, class definitions, and the forward pass. Then trace the data flow from input to output, explaining how tensors are transformed at each step. Finally, discuss the overall purpose and any design trade-offs.

Pro tip: Verbalize your thought process as you navigate the code, and don't hesitate to ask clarifying questions about the context or requirements. This demonstrates adaptability and a collaborative mindset.

1. Skim and Identify Structure

Quickly scan the file to locate key sections: imports, class definitions, __init__ methods, and forward methods. Note any unfamiliar libraries or patterns.

2. Trace the Data Flow

Start from the input and follow the data through each operation, explaining how tensors are reshaped, transformed, and passed between layers or functions.

3. Explain the Purpose

Summarize what the code is doing overall: is it a model, a training loop, a data pipeline? Relate it to common PyTorch patterns.

4. Discuss Trade-offs and Ambiguities

Point out any design choices, potential inefficiencies, or unclear parts, and suggest alternatives or ask for clarification.

Key Points to Mention

  • PyTorch tensor operations and shape transformations
  • Model architecture (e.g., layers, activations, loss functions)
  • Data flow from input to output, including any preprocessing
  • Training vs. inference mode differences
  • Potential performance bottlenecks or design trade-offs
  • Assumptions made and areas needing clarification

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