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Openai·Software Engineer·Technical Phone Screen·Senior

SeniorPrefer not to say
Jun 2026Remote

Summary

OpenAI SWE interview centered on a scheduling problem for a data labeling platform, with multiple follow-up parts that got progressively harder. The core question is apparently making the rounds right now and caught a lot of people off guard on the prefix-balance constraint.

Questions Asked (4)

Q1

Design a scheduler for a data labeling platform with t tasks, m models, and h human labelers. Produce a schedule as a list of (task, model, human) assignments where each human participates in at least k tasks and each (task, human) pair appears at most once.

Algorithms & Data StructuresSystem Design
Author's notes

Part 1 felt manageable.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the problem constraints and objectives, then model it as a bipartite matching or flow problem to ensure each human gets at least k tasks and no duplicate (task, human) pairs. Propose a greedy or flow-based algorithm, analyze its complexity, and discuss scalability and potential optimizations for large-scale data labeling.

Pro tip: Emphasize the importance of load balancing and quality control in data labeling; mention that in practice, you might assign multiple models per task for cross-validation and use human labelers for ambiguous cases, which can be modeled by adjusting the constraints.

1. Clarify Requirements and Constraints

Ask questions to understand the scale (t, m, h), whether tasks can be assigned to multiple humans, if models are required for each task, and if there are any additional constraints like human expertise or model accuracy. Confirm that each human must participate in at least k tasks and each (task, human) pair appears at most once.

2. Model as a Graph Problem

Represent tasks, models, and humans as nodes in a tripartite graph or reduce to a bipartite matching problem between tasks and humans, with models as intermediaries. Formulate as a flow network where source connects to tasks, tasks to models, models to humans, and humans to sink, with capacities ensuring the constraints.

3. Design Algorithm

Propose an algorithm such as max-flow with lower bounds to ensure each human gets at least k tasks, or a greedy approach that iteratively assigns tasks to humans while avoiding duplicates. Discuss how to handle the model dimension, e.g., by first assigning models to tasks or integrating them into the flow.

4. Analyze Complexity and Scalability

Analyze time and space complexity of the proposed algorithm. For large-scale systems, discuss distributed or approximation algorithms, and how to handle failures or dynamic changes (e.g., humans dropping out).

5. Discuss Practical Considerations

Mention real-world factors like human expertise, task difficulty, model confidence, and quality control. Suggest how to incorporate these into the scheduler, e.g., by weighting edges or adding constraints, and how to evaluate the schedule's effectiveness.

Key Points to Mention

  • Bipartite matching or max-flow formulation with lower bounds to satisfy the at-least-k constraint.
  • Handling the tripartite relationship (task, model, human) by reducing to bipartite or using flow networks.
  • Ensuring no duplicate (task, human) pairs by capacity constraints or assignment tracking.
  • Complexity analysis: polynomial time for flow algorithms, but may be too slow for very large t, m, h; consider heuristics.
  • Scalability: distributed scheduling, batch processing, and incremental updates.
  • Quality control: assigning multiple models or humans per task for cross-validation, and using human labelers for ambiguous cases.

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

Q2

Extend the scheduler so that for every prefix of the output schedule, the difference between the maximum and minimum number of times any model has been assigned to a given task is at most 1. Same condition must hold for human labelers per task.

Algorithms & Data StructuresTechnical Trade-offs
Author's notes

This is where I got stuck.

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AI HintsAI Generated

Suggested Approach

First, clarify the problem constraints and define the fairness condition precisely. Then, propose a greedy algorithm that maintains counts for each model-task and human-task pair, ensuring that at each step the assignment keeps the max-min difference ≤ 1. Finally, discuss data structures and trade-offs, such as using priority queues or round-robin with counters.

Pro tip: Emphasize that the condition must hold for every prefix, not just the final schedule, so online greedy assignment is necessary. Mention that this is similar to load balancing with a strict fairness constraint.

1. Clarify the problem

Restate the requirement: for any prefix of the schedule, the difference between the maximum and minimum number of assignments for any model to a given task is at most 1, and similarly for human labelers per task. Confirm that assignments are made sequentially and the condition must hold at every step.

2. Design a greedy algorithm

Propose a greedy approach: at each step, for the chosen task, assign the model (and human) that currently has the minimum count for that task, breaking ties arbitrarily. This ensures the max-min difference never exceeds 1.

3. Choose efficient data structures

Use a min-heap or balanced BST per task to track counts of models and humans, allowing O(log n) selection of the minimum. Alternatively, maintain a queue for round-robin if counts are balanced.

4. Analyze correctness and complexity

Prove that the greedy choice maintains the invariant: after each assignment, the max-min difference for that task remains ≤ 1. Analyze time complexity: O(log n) per assignment with heaps, or O(1) with round-robin if applicable.

5. Discuss trade-offs and extensions

Consider trade-offs: strict fairness may reduce throughput or require more state. Discuss handling dynamic changes, like adding new models or tasks, and whether the condition can be relaxed.

Key Points to Mention

  • The condition must hold for every prefix, so online greedy assignment is necessary.
  • Use a min-heap or priority queue to efficiently select the least-assigned model/human for a task.
  • Prove correctness by induction: after each assignment, the max-min difference remains ≤ 1.
  • Time complexity: O(log n) per assignment with heaps, or O(1) with round-robin if counts are balanced.
  • Trade-offs: fairness vs. throughput, and potential need for tie-breaking strategies.
  • Consider scalability: handling many tasks and models/humans efficiently.

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

Q3

Further extend the balanced scheduler so that for each human labeler, the distribution of models they are paired with is also as even as possible across all (model, human) combinations.

Algorithms & Data StructuresTechnical Trade-offs
Author's notes

They flagged this as significantly harder than the previous part, which is accurate.

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AI HintsAI Generated

Suggested Approach

First, clarify the existing balanced scheduler's constraints and objectives, then model the new requirement as a bipartite graph where edges represent (model, human) assignments with weights reflecting current load. Propose an algorithm that extends the current balancing mechanism, such as a min-cost max-flow or a greedy approach with backtracking, ensuring both human load and model distribution are balanced.

Pro tip: Emphasize that perfect balance may be impossible due to constraints, so discuss trade-offs and propose a solution that minimizes variance while maintaining other constraints. Also, mention the importance of defining a clear metric for 'evenness' (e.g., standard deviation of assignments per model per human).

1. Clarify Requirements and Constraints

Ask clarifying questions to understand the existing scheduler, the definition of 'even distribution', and any constraints like availability or model requirements. Confirm whether the goal is to minimize the maximum difference or variance across (model, human) pairs.

2. Model as a Bipartite Graph

Represent humans and models as nodes, with edges representing possible assignments. Assign weights or capacities to edges to track current assignment counts, and define the objective as minimizing the deviation from a perfectly even distribution.

3. Choose an Algorithmic Approach

Consider algorithms like min-cost max-flow with costs based on current load, or a greedy algorithm that iteratively assigns the least-assigned model to each human. Discuss complexity and scalability.

4. Handle Trade-offs and Edge Cases

Address potential conflicts with existing balancing goals (e.g., human workload) and propose a multi-objective optimization or weighted approach. Discuss edge cases like when the number of models is not divisible by humans.

5. Validate and Test

Propose a testing strategy, such as simulation with random inputs, to verify that the distribution is as even as possible and that other constraints are satisfied. Mention metrics like standard deviation or entropy.

Key Points to Mention

  • Bipartite graph representation of (model, human) assignments
  • Min-cost max-flow or assignment problem algorithms
  • Definition of 'even distribution' (e.g., minimize variance or max difference)
  • Trade-offs with existing constraints (human workload, model requirements)
  • Scalability and complexity considerations
  • Testing and validation with metrics like standard deviation

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

Q4

Adapt the scheduler to a streaming setting where new tasks arrive each day, the schedule must be updated incrementally without restarting from scratch, and each human can handle at most one task per day.

System DesignAlgorithms & Data Structures
Author's notes

Didn't get here in the interview.

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AI HintsAI Generated

Suggested Approach

First, clarify the problem constraints and define the incremental update requirements. Then, propose a data structure that supports efficient insertion and reassignment, such as a priority queue or segment tree, and discuss how to maintain the one-task-per-day constraint. Finally, analyze the time complexity of updates and compare with a full recomputation to justify the incremental approach.

Pro tip: Emphasize that incremental updates should be O(log n) or O(1) per new task to handle high throughput, and mention that you would monitor for schedule drift or starvation to ensure fairness over time.

1. Clarify requirements and constraints

Ask about the scale of tasks and humans, the definition of 'incremental', and whether tasks have deadlines or priorities. Confirm that each human can handle at most one task per day and that new tasks arrive daily.

2. Choose an appropriate data structure

Propose a data structure that allows efficient insertion and retrieval of the best available human for a task, such as a priority queue keyed by human availability or a segment tree over days. Consider using a min-heap of humans sorted by next available day.

3. Design the incremental update algorithm

For each new task, assign it to the earliest available human without violating the one-task-per-day constraint. Update the human's next available day and adjust the data structure accordingly. If reassignment is needed, use a local search or augmenting path approach.

4. Analyze complexity and trade-offs

Discuss the time complexity of adding a task (e.g., O(log n) with a heap) and compare with full recomputation (O(n log n) or worse). Mention space complexity and potential bottlenecks.

5. Address edge cases and scalability

Cover scenarios like multiple tasks arriving simultaneously, humans becoming unavailable, or tasks with deadlines. Suggest monitoring and fallback strategies, such as periodic rebalancing or batching updates.

Key Points to Mention

  • Incremental update vs. full recomputation: emphasize efficiency gains
  • Data structures: priority queue, segment tree, or balanced BST for availability tracking
  • Constraint handling: one task per human per day, avoiding conflicts
  • Time complexity: aim for O(log n) per insertion/update
  • Fairness and load balancing: prevent starvation and ensure even distribution
  • Scalability: handle high arrival rates and large numbers of humans/tasks

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