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BlackRock·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say
May 2026

Summary

BlackRock software engineer interview with a coding round that was basically a mini design-plus-implementation problem. One question, but it had enough layers to keep you busy for the whole session.

Questions Asked (1)

Q1

Design and implement a simplified Paint editor class that supports drawing, erasing, and flood-fill operations on a 2D canvas, along with full undo and redo functionality.

System DesignAlgorithms & Data StructuresTechnical Trade-offs
Author's notes

The fill part was fine, BFS/DFS flood fill is pretty standard.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and constraints, then outline a clean class design with separate concerns for canvas state, operations, and history management. Discuss algorithms for drawing, erasing, and flood fill, and explain how to implement undo/redo using command pattern or state snapshots, weighing trade-offs.

Pro tip: Demonstrate awareness of memory vs. performance trade-offs: for example, using command pattern with inverse operations saves memory but complicates erasing, while snapshotting is simpler but costly for large canvases. Propose a hybrid or adaptive approach based on expected usage.

1. Clarify Requirements and Constraints

Ask about canvas size, pixel representation, expected operation frequency, memory limits, and whether undo/redo should be unlimited. This shows you think about real-world constraints before coding.

2. Design the Core Data Structures

Define the Canvas class with a 2D array (or 1D array for efficiency) to store pixel colors. Consider using a sparse representation if the canvas is large and mostly empty.

3. Implement Drawing, Erasing, and Flood Fill

For drawing/erasing, update pixels in a line or shape. For flood fill, use BFS/DFS to change connected pixels of the same color, being mindful of recursion depth and using an iterative approach.

4. Implement Undo/Redo Mechanism

Choose between command pattern (store operations and their inverses) or snapshotting (store canvas states). Discuss trade-offs: command pattern is memory-efficient but complex for flood fill; snapshots are simple but memory-heavy.

5. Optimize and Discuss Trade-offs

Propose optimizations like compression of snapshots, batching operations, or using a limited history buffer. Explain how you would test and scale the solution.

Key Points to Mention

  • Command pattern with inverse operations for undo/redo, and how to handle flood fill's inverse (e.g., storing affected pixels).
  • Snapshot-based undo/redo using deep copies or persistent data structures, and memory implications.
  • Flood fill algorithm: BFS vs DFS, iterative vs recursive, and handling edge cases like same-color fill.
  • Canvas representation: dense 2D array vs sparse map, and performance trade-offs for large canvases.
  • Thread safety and concurrency considerations if the editor is used in a multi-threaded environment.
  • Testing strategy: unit tests for each operation, undo/redo sequences, and stress tests for large canvases.

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