Robinhood·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Jun 2026

Summary

Robinhood coding round, one problem the whole time. It's a proportional allocation question with rounding logic, which sounds like basic math until you're actually implementing it under pressure.

Questions Asked (1)

Given a total order quantity Q and a list of users with fractional ownership weights that sum to 1, allocate Q proportionally across users, round each allocation to the configured lot size, then distribute any leftover units to users with the largest fractional remainders so the total always equals Q. Return the per-user allocations.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

My first instinct was to just multiply and round and call it done.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the problem constraints and edge cases, then outline a two-phase algorithm: first compute raw allocations and round down to the nearest lot size, then distribute the remaining units based on fractional remainders. Emphasize correctness, efficiency, and handling of edge cases like zero lot size or negative Q.

Pro tip: Mention that you would use a max-heap or sorting for the remainder distribution to achieve O(n log n) time, and discuss how to handle ties deterministically (e.g., by user ID) to ensure reproducibility.

1. Clarify requirements and edge cases

Ask about lot size (e.g., integer, fractional), whether Q is always divisible by lot size, and how to handle ties. Confirm that weights sum to 1 and Q is non-negative.

2. Compute raw allocations and round down

For each user, calculate raw allocation = weight * Q. Round down to the nearest multiple of lot size (floor division). Track the fractional remainder after rounding down.

3. Calculate leftover units

Sum the rounded-down allocations and subtract from Q to get the leftover units. Ensure leftover is non-negative and a multiple of lot size.

4. Distribute leftovers by largest remainders

Sort users by fractional remainder descending (break ties by user ID or stable order). Assign one lot size to each user in order until leftover is exhausted.

5. Return allocations and discuss complexity

Return the final per-user allocations. Analyze time complexity (O(n log n) due to sorting) and space complexity (O(n)). Mention potential optimizations like using a heap for large n.

Key Points to Mention

Proportional allocation using weights and total quantity Q
Rounding down to lot size and tracking fractional remainders
Largest remainder method for distributing leftovers
Handling ties deterministically (e.g., by user ID)
Edge cases: zero lot size, Q not divisible by lot size, negative Q
Time and space complexity, and potential optimizations

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