Amazon·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Jun 2026

Summary

Amazon SWE interview with a coding question around lazy evaluation. Pretty niche concept to get asked about, not the typical array manipulation stuff I'd been grinding.

Questions Asked (1)

Implement a Lazy Array that defers element computation until the element is actually accessed. Include a method to access elements and a preprocessing method that only computes what's needed and hasn't been computed yet.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

I'd used lazy evaluation conceptually before but never had to build it from scratch under pressure.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and constraints, then design a class that stores a generator function and a cache. Implement the access method to compute and cache the value only if not already cached, and the preprocessing method to compute all uncached elements up to a given index or all elements. Discuss trade-offs between memory and computation time.

Pro tip: Mention that lazy evaluation is particularly useful when computations are expensive and not all elements are needed, but be aware of potential memory overhead from caching and thread-safety concerns if accessed concurrently.

1. Clarify Requirements

Ask about expected operations, thread-safety, memory constraints, and whether the preprocessing method should compute all elements or up to a certain index.

2. Design Data Structures

Use an array or list to store computed values (with a sentinel for uncomputed) and a function or generator to compute elements on demand.

3. Implement Access Method

Check if the element at the given index is already computed; if not, compute it using the generator, store it, and return it.

4. Implement Preprocessing Method

Iterate through indices that haven't been computed yet (up to a specified limit or all) and compute and store their values.

5. Discuss Trade-offs and Optimizations

Talk about time vs. space trade-offs, potential for parallel computation, and handling of edge cases like out-of-bounds access.

Key Points to Mention

Lazy evaluation defers computation until needed, saving time and resources when not all elements are accessed.
Caching computed values avoids redundant computation but increases memory usage.
The preprocessing method should only compute elements that are not yet cached, possibly up to a given index.
Thread-safety considerations: use locks or atomic operations if the array may be accessed concurrently.
Edge cases: handling out-of-bounds indices, null generator, and ensuring the generator is pure or idempotent.
Trade-offs: memory vs. computation time, and potential for parallelizing the preprocessing step.

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