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SoFi·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
May 2026

Summary

SoFi full-stack screen with a pretty involved coding problem centered on probability and simulation. Not your typical LeetCode grind, which threw me off a bit.

Questions Asked (1)

Q1

Given a run-tracking system with Run and RunCollection classes for an obstacle course, implement a function that uses Monte Carlo simulation (10,000 trials) to estimate the probability that an in-progress run will finish at or below the current personal best. Each trial should fill in the remaining obstacles by randomly sampling from historical run data, then compare the simulated total against the personal best.

Algorithms & Data StructuresTechnical Trade-offs
Author's notes

I knew what Monte Carlo meant in theory but had never actually coded one under pressure.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the data model and simulation parameters, then outline a Monte Carlo algorithm that samples remaining obstacles from historical data and compares simulated totals to the personal best. Emphasize code structure, efficiency, and statistical correctness while discussing potential trade-offs.

Pro tip: Mention that you would seed the random number generator for reproducibility and use vectorized operations or parallelization to handle 10,000 trials efficiently, showing awareness of performance in production systems.

1. Clarify Requirements and Assumptions

Confirm the structure of Run and RunCollection, how historical data is stored, and what 'in-progress' means (e.g., obstacles completed so far). Ask about performance constraints and whether the personal best is fixed or dynamic.

2. Design the Simulation Algorithm

Outline a Monte Carlo loop: for each trial, copy the current run's completed obstacles, then for each remaining obstacle, randomly sample a completion time from historical data (e.g., from all runs or similar runs). Sum the times to get a simulated total.

3. Implement Efficient Sampling

Describe how to sample from historical data efficiently: precompute distributions per obstacle, use random.choice or numpy.random.choice, and consider caching or pre-aggregation to avoid repeated scans.

4. Compare and Estimate Probability

After each trial, compare the simulated total to the personal best. Count successes and divide by the number of trials (10,000) to estimate the probability. Optionally compute confidence intervals.

5. Discuss Trade-offs and Optimizations

Talk about time/space complexity, potential biases in sampling (e.g., using all historical data vs. recent runs), and optimizations like parallelization or early stopping if the probability converges.

Key Points to Mention

  • Monte Carlo simulation basics: random sampling, trial count, and probability estimation.
  • Data structures: efficient storage and retrieval of historical obstacle times (e.g., hash maps or arrays).
  • Sampling methodology: with/without replacement, stratification, or bootstrapping to avoid bias.
  • Performance considerations: time complexity O(trials * remaining obstacles), memory usage, and potential parallelization.
  • Statistical validity: ensuring independence of samples, handling missing data, and confidence intervals.
  • Code quality: modular design, clear separation of simulation logic from data access, and testability.

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