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eBay·Software Engineer·Technical Phone Screen·Senior

Senior
Apr 2026

Summary

Did a technical phone screen for a software engineer role at eBay that was basically one big design-and-implement question about bitmap block management. More depth than I expected for a phone round, they really wanted to dig into data structure tradeoffs and not just get a working solution.

Questions Asked (1)

Q1

Design and implement a BitmapBlock class that manages a fixed-size bitmap of allocation slots, supporting construction, contiguous block allocation from a given index, searching for free contiguous blocks, and counting the number of maximal free runs.

Algorithms & Data StructuresSystem DesignTechnical Trade-offs
Author's notes

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AI HintsAI Generated

Suggested Approach

Start by clarifying the requirements and constraints, then design the class with a clear internal representation (e.g., a bit array or integer array). Implement each method with efficient bitwise operations, and discuss trade-offs between time and space complexity. Finally, walk through an example to demonstrate correctness.

Pro tip: Mention that you would use a hierarchical bitmap or a tree structure if the bitmap size is very large, to improve search performance. Also, emphasize the importance of unit tests for edge cases like full/empty bitmaps and boundary indices.

1. Clarify requirements and constraints

Ask about the expected size of the bitmap, the frequency of operations, and whether thread safety is required. This helps tailor the design.

2. Choose internal representation

Decide between a simple boolean array, a bit-packed integer array, or a more advanced structure like a segment tree. Justify your choice based on performance needs.

3. Implement core operations

For allocation, check if the requested range is free and mark it allocated. For searching, scan for contiguous free blocks. For counting maximal free runs, iterate through the bitmap and count transitions from free to allocated.

4. Optimize with bitwise operations

Use bitwise AND, OR, and shifts to efficiently check and set ranges. For example, to check if a range is free, create a mask and compare with the current state.

5. Analyze complexity and trade-offs

Discuss time complexity for each operation (e.g., O(n) for scanning, O(1) for allocation if using bitwise checks) and space complexity. Mention alternative data structures for better performance.

Key Points to Mention

  • Bitwise operations for efficient range checks and updates
  • Time and space complexity of each operation
  • Handling edge cases: full bitmap, empty bitmap, out-of-bounds indices
  • Trade-offs between simple bit array and hierarchical structures
  • Thread safety considerations if concurrent access is expected
  • Testing strategy including unit tests for boundary conditions

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