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Coinbase·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say
Jun 2026Remote

Summary

Coinbase backend interview focused on extending a recipe management system, which sounds chill until you realize they want you to reason through indexing tradeoffs on the fly. Two feature additions plus a design discussion, all in one round.

Questions Asked (2)

Q1

You're given a recipe management system with basic CRUD operations backed by a hash map. Add a search function that does case-insensitive substring matching across recipe names and returns all matching recipes.

Algorithms & Data StructuresTechnical Trade-offsSystem Design
Author's notes

My first instinct was to just lowercase everything on each scan and call it done.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements: case-insensitive substring match on recipe names, return all matches, and consider performance expectations. Then propose a simple linear scan over the hash map's values, but discuss trade-offs like time complexity and potential optimizations such as maintaining an index or using a trie. Finally, outline the implementation with clean code and edge cases.

Pro tip: Mention that while a linear scan is O(n) per query, for a recipe management system the number of recipes is likely small, so it's acceptable; but if search becomes frequent, consider building an inverted index or using a case-insensitive data structure. This shows you balance simplicity with scalability.

1. Clarify Requirements

Confirm that search should be case-insensitive, match substrings anywhere in the name, and return all matching recipes. Ask about expected data size and query frequency to inform design.

2. Choose Data Structure & Algorithm

Since the system uses a hash map, iterate over its values and check if the lowercased recipe name contains the lowercased query. Discuss time complexity O(n*m) where n is number of recipes and m is average name length.

3. Consider Optimizations

If performance is critical, propose alternatives like maintaining a separate index (e.g., inverted index of words) or using a trie for prefix/substring searches, but note added complexity and memory trade-offs.

4. Implement & Handle Edge Cases

Write the search function, ensuring it handles empty query (return all or none?), null inputs, and returns an empty list if no matches. Use efficient string operations.

5. Test & Validate

Walk through test cases: exact match, partial match, different cases, no match, and empty query. Verify correctness and discuss potential improvements.

Key Points to Mention

  • Time complexity: O(n*m) for linear scan, where n is number of recipes and m is average name length.
  • Space complexity: O(1) extra space for linear scan, or O(n) if building an index.
  • Case-insensitive matching: convert both query and recipe names to lowercase (or use case-insensitive comparison).
  • Trade-offs: simplicity vs. performance; when to optimize (e.g., if search is frequent or dataset large).
  • Edge cases: empty query, null inputs, no matches, and special characters.
  • Potential optimizations: inverted index, trie, or caching frequent queries.

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

Q2

Add a listRecipes function that returns all recipes sorted by either name or size. Walk through how you'd implement it and whether you'd maintain a sorted structure or sort on demand.

Algorithms & Data StructuresTechnical Trade-offsData Modeling
Author's notes

Went with sort-on-demand first since it's simpler and I didn't want to overcomplicate it.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the requirements: what fields define 'name' and 'size', expected data volume, read/write ratio, and whether the sort order needs to be stable. Then present a clean implementation that sorts on demand using a comparator, and discuss the trade-offs of maintaining a sorted structure versus sorting on demand, considering performance, memory, and complexity.

Pro tip: Mention that for small to medium datasets, sorting on demand is simpler and less error-prone, but for large datasets with frequent reads and infrequent writes, maintaining a sorted structure (like a balanced BST or sorted array with binary search insertion) can be more efficient. Also, note that Coinbase likely values scalability and clean code, so emphasize choosing the right approach based on data characteristics.

1. Clarify Requirements

Ask about the definition of 'name' and 'size', expected data volume, frequency of reads vs writes, and whether the list needs to be sorted in-place or can return a new list.

2. Design the Function Signature

Define the function to accept a sort key (e.g., 'name' or 'size') and an optional order (ascending/descending). Return a list of recipes sorted accordingly.

3. Implement Sorting On Demand

Use a comparator or key function to sort the list when the function is called. This is simple and works well if the data doesn't change often or the list is small.

4. Discuss Maintaining a Sorted Structure

Explain that if the recipe list is large and read frequently, you could maintain a sorted data structure (e.g., balanced BST, skip list, or sorted array with binary search insertion) to achieve O(log n) insertion and O(1) or O(log n) retrieval.

5. Compare Trade-offs

Weigh the pros and cons: sorting on demand is simpler, uses less memory, and avoids update overhead; maintaining sorted order gives faster reads but complicates writes and may use more memory. Choose based on access patterns.

Key Points to Mention

  • Time complexity: sorting on demand is O(n log n) per call; maintaining sorted order can reduce read time to O(1) or O(log n) but increases write time.
  • Space complexity: sorting on demand may require O(n) extra space for the sorted copy; maintaining a sorted structure may require additional pointers or overhead.
  • Stability: if recipes have equal names or sizes, ensure the sort is stable or define a secondary sort key.
  • Immutability: consider whether to return a new sorted list or sort in-place, and the implications for thread safety.
  • Scalability: for large datasets, consider external sorting or database-level sorting if data is persisted.
  • Testing: include edge cases like empty list, single item, duplicate keys, and invalid sort keys.

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