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Google·Software Engineer·Onsite - System Design / Architecture·Senior

SeniorPrefer not to say
May 2026

Summary

Google system design round for a software engineering role. The question was a single meaty design problem that took the whole session, and I left feeling like I'd only scratched the surface of what they were actually looking for.

Questions Asked (1)

Q1

Design a mutable ordered sequence with a marker (cursor/bookmark) that points to one element. Support inserting and removing contiguous blocks at arbitrary positions, moving a block from one position to another, and repositioning the marker. Walk through your API, data model, and how the marker should behave when edits happen near or around it.

System DesignData ModelingTechnical Trade-offs
Author's notes

The marker behavior is where I got tripped up.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and defining a clear API for the sequence and marker operations, then propose a data model (e.g., balanced tree or skip list) that supports efficient block operations and marker tracking. Discuss how the marker behaves under edits, covering edge cases like insertions/removals at the marker position, and analyze trade-offs between different implementations.

Pro tip: Explicitly define marker semantics (e.g., does it stick to the element or the position?) and justify your choice based on typical use cases like text editors or collaborative editing. This shows you think about real-world implications and user experience.

1. Clarify Requirements and Scope

Ask questions to understand expected operations, performance needs, and marker behavior. Confirm whether the sequence is large, if concurrent access is needed, and what 'block' means (contiguous elements).

2. Define the API

Specify methods for inserting, removing, and moving blocks, as well as marker operations (get, set, move). Include parameters like position indices and block sizes.

3. Choose a Data Model

Propose a data structure that supports efficient block operations, such as a balanced binary tree (e.g., rope or implicit treap) or a skip list. Explain how it enables O(log n) insertions, deletions, and moves.

4. Define Marker Behavior

Decide how the marker updates when edits occur: does it stay attached to the same element, shift with insertions/deletions before it, or become invalid if its element is removed? Specify rules for each operation.

5. Analyze Trade-offs and Edge Cases

Discuss performance implications, memory overhead, and complexity of implementation. Cover edge cases like empty sequence, marker at boundaries, and moving a block that contains the marker.

Key Points to Mention

  • Choice of data structure (e.g., balanced tree, skip list) and its impact on time complexity for block operations and marker updates.
  • Marker semantics: whether it tracks an element (sticky) or a position (index-based), and how that affects behavior during insertions, deletions, and moves.
  • Handling of edge cases: marker at the start/end, marker within a moved block, and operations on empty sequences.
  • API design considerations: clarity, consistency, and whether operations return new markers or update in place.
  • Trade-offs between different implementations: simplicity vs. performance, memory usage, and concurrency support.
  • Real-world applications (e.g., text editors, collaborative editing) to justify design decisions.

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