← OpenAI Interview Insights

OpenAI·Software Engineer·Technical Phone Screen·Senior

SeniorPrefer not to say
Jul 2026

Summary

Got a coding problem at OpenAI for a software engineer role that was deceptively tricky to think through cleanly. The core problem was manageable but the follow-up about prefix-balanced scheduling added a whole layer I wasn't fully prepared for.

Questions Asked (2)

Q1

Design a scheduling system for a data labeling platform with tasks, AI models, and human annotators. Given counts for each entity type and a minimum assignment threshold k, return a list of [task, model, human] assignments such that every human appears in at least k assignments and no human is assigned the same task more than once.

Algorithms & Data StructuresSystem Design
Author's notes

Spent the first few minutes just making sure I understood the constraints, which was the right call.

Create a free account to read the full note

AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and constraints, then model the problem as a bipartite graph between humans and tasks with models as intermediaries. Propose a greedy or flow-based algorithm that ensures each human gets at least k assignments without duplicate tasks, and discuss scalability and fairness.

Pro tip: Emphasize the importance of load balancing and avoiding starvation; consider using a round-robin or priority-based approach to ensure all humans meet the threshold while maintaining quality.

1. Clarify Requirements and Constraints

Ask about the scale (number of tasks, models, humans), the value of k, whether assignments can be repeated across humans, and if there are any constraints on model-task compatibility or human expertise.

2. Model the Problem

Represent the problem as a bipartite graph where humans need to be matched to tasks, with models as possible intermediaries. Define the goal as finding a set of triples (task, model, human) such that each human appears in at least k triples and no human-task pair repeats.

3. Design an Algorithm

Propose a greedy algorithm: iterate over tasks, assign each task to a model and then to humans who haven't been assigned that task, prioritizing humans with fewer assignments. Alternatively, use a max-flow formulation with lower bounds on human nodes.

4. Ensure Correctness and Efficiency

Analyze the algorithm's complexity and prove that it meets the constraints. Discuss potential bottlenecks and how to handle large-scale data, such as using distributed processing or approximation algorithms.

5. Discuss Extensions and Trade-offs

Consider extensions like dynamic task arrival, human availability, or quality control. Discuss trade-offs between fairness, throughput, and cost.

Key Points to Mention

  • Bipartite matching or flow network formulation with lower bounds
  • Greedy assignment with priority queue for load balancing
  • Handling the no-duplicate constraint by tracking assigned tasks per human
  • Scalability considerations: distributed scheduling, sharding, or batch processing
  • Fairness and avoiding starvation of human annotators
  • Potential use of AI models for pre-labeling or quality checks

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

Q2

Follow-up: modify your solution to return a prefix-balanced schedule, where at every prefix of the output the task-model usage counts differ by at most 1, and each human's per-model usage counts also differ by at most 1.

Algorithms & Data StructuresTechnical Trade-offs
Author's notes

This is where I started struggling.

Create a free account to read the full note

AI HintsAI Generated

Suggested Approach

First, clarify the exact constraints: what does 'prefix-balanced' mean for task-model usage and per-human per-model usage? Then, propose a greedy algorithm that at each step selects the next task-model assignment that maintains the balance conditions, possibly using a priority queue or round-robin approach. Finally, analyze the time complexity and discuss trade-offs between strict balance and other objectives like minimizing makespan.

Pro tip: Demonstrate awareness that strict prefix-balance might conflict with other scheduling goals, and be ready to discuss relaxations or alternative definitions of balance. Also, mention that you would test with small cases to verify the invariant holds at every prefix.

1. Clarify the problem and constraints

Ask questions to ensure you understand what 'prefix-balanced' means: does it apply to the difference between max and min usage counts across all task-models at each prefix? Similarly for each human's per-model usage? Confirm if the schedule is built incrementally and if the balance must hold after each assignment.

2. Design a greedy algorithm

At each step, consider the current usage counts. Choose the next task-model assignment that reduces the maximum imbalance, possibly by selecting the task-model with the lowest usage count relative to others. Ensure that for each human, their per-model counts also remain balanced by possibly rotating models for that human.

3. Prove correctness and analyze complexity

Argue that the greedy choice maintains the invariant that at every prefix, the max-min difference for task-model usage is at most 1, and similarly for each human's per-model usage. Analyze the time complexity, likely O(n log m) with a priority queue, where n is number of tasks and m is number of models.

4. Discuss trade-offs and edge cases

Consider cases where the number of tasks is not divisible by the number of models, or where human constraints conflict. Discuss if the greedy approach always works or if backtracking is needed. Mention potential relaxations if strict balance is impossible.

5. Test with examples

Walk through a small example to demonstrate the algorithm and verify that the prefix-balance condition holds at each step. Show how the counts evolve and remain within 1.

Key Points to Mention

  • Definition of prefix-balance: at every prefix, the difference between max and min usage counts for task-models is ≤1, and for each human's per-model usage is ≤1.
  • Greedy strategy: always assign the next task to the model (for that task) that currently has the lowest usage count, ensuring balance.
  • Use of priority queues or sorted lists to efficiently select the next assignment.
  • Time complexity: O(n log m) where n is number of tasks and m is number of models, if using a heap.
  • Proof of invariant: by induction, the greedy choice maintains the balance condition at each step.
  • Trade-offs: strict prefix-balance may lead to suboptimal overall makespan or other metrics; discuss if relaxation is acceptable.

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