Snapchat·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

Jun 2026Remote

Summary

Snapchat SWE interview that went deep on custom data structures and functional programming patterns. One question but it had a lot of layers and I was not fully prepared for the generics conversation that followed the coding part.

Questions Asked (1)

You have a hand-built List backed by a dynamic array with standard operations (add, get, set, remove, size) and an Iterator (hasNext, next). Implement a map(fn) method that applies a given function to each element and returns a new List of the results, using only the Iterator and not touching the backing array directly. Then discuss generic typing where input and output types can differ, eager vs. lazy evaluation, and how laziness interacts with source list mutation.

Algorithms & Data StructuresTechnical Trade-offsSystem Design

Author's notes

The coding part was fine.

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AI HintsAI Generated

Suggested Approach

Start by writing the map method using only the Iterator, then explain how generics allow different input and output types. Finally, discuss eager vs. lazy evaluation and the implications of laziness with source list mutation.

Pro tip: Mention that lazy evaluation can cause unexpected behavior if the source list is mutated after map is called, and suggest documenting or defending against it.

1. Implement map with Iterator

Create a new List, obtain an Iterator from the source list, and iterate while hasNext() is true, applying fn to each element and adding the result to the new list.

2. Discuss generic typing

Explain that map should be generic: <R> List<R> map(Function<T, R> fn), allowing input type T and output type R to differ.

3. Compare eager vs. lazy evaluation

Eager evaluation applies fn immediately and returns a new list; lazy evaluation returns a view that applies fn on demand, saving memory and computation if not all elements are needed.

4. Analyze laziness and mutation

If lazy, the view reflects changes to the source list after map is called, which can lead to inconsistent or surprising results; discuss trade-offs and possible solutions like copying or documenting behavior.

Key Points to Mention

Iterator usage: only hasNext() and next(), no direct array access.
Generic method signature with type parameters for input and output.
Eager evaluation: immediate computation, new list, no side effects from later mutations.
Lazy evaluation: deferred computation, potential memory savings, but view depends on source.
Mutation interaction: lazy view sees changes, eager snapshot does not.
Trade-offs: performance, memory, predictability, and thread-safety considerations.

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