Plaid·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

May 2026Remote

Summary

Plaid SWE interview with a meaty algorithmic problem that had more moving parts than I expected. The core idea isn't too bad but they wanted the full package: proof of correctness, complexity analysis, working code, and tests, all in one shot.

Questions Asked (1)

Given an array of item prices in a checkout line, apply a discount rule where each item's discount equals the price of the first subsequent item that costs less than or equal to it. Return the array of final prices and the total savings. Solve in O(n) time, prove correctness, analyze complexity, and write code with tests.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

I recognized the monotonic stack pattern pretty fast, which was a relief.

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AI HintsAI Generated

Suggested Approach

Clarify the problem and edge cases, then propose a monotonic stack solution that computes the next smaller-or-equal element in O(n) time. Walk through the algorithm with a small example, prove correctness via loop invariants, analyze time and space complexity, and finally write clean code with tests covering edge cases.

Pro tip: Emphasize that the discount is the first subsequent item with price <= current, not the global minimum, and that a monotonic stack efficiently finds this 'next smaller or equal' element. Also, proactively discuss trade-offs like space complexity and potential integer overflow when summing savings.

1. Clarify requirements and edge cases

Confirm that the discount is the price of the first subsequent item that is <= the current item, and that if none exists, the discount is 0. Discuss edge cases: empty array, single item, all increasing, all decreasing, duplicates.

2. Design the algorithm

Use a monotonic increasing stack (indices) to track items waiting for a smaller-or-equal price. Iterate through prices; for each price, pop indices where price <= stack top's price, assign discount, and push current index.

3. Prove correctness

Argue that the stack maintains indices of items whose next smaller-or-equal element hasn't been found, in increasing order of price. When a new price is <= stack top's price, it is the first such element for that index, so the discount is correctly assigned.

4. Analyze complexity

Each index is pushed and popped at most once, so time is O(n). Space is O(n) for the stack and output array.

5. Implement and test

Write code with clear variable names, handle edge cases, and include unit tests for typical and edge scenarios. Optionally, discuss alternative approaches like brute force O(n^2) and why the stack is better.

Key Points to Mention

Monotonic stack technique for next smaller or equal element
Time complexity O(n) and space complexity O(n)
Correctness proof using loop invariant: stack contains indices with no smaller-or-equal element yet, in increasing price order
Handling of edge cases: empty array, no discount, duplicates
Trade-offs: stack space vs. brute force time, potential integer overflow in total savings
Testing strategy: unit tests for various inputs, including large arrays to verify linear time

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