eBay·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

May 2026

Summary

eBay coding interview with a grid simulation problem that felt deceptively tricky once gravity kicked in. Not a lot of context given upfront, just the problem and a blank screen.

Questions Asked (1)

Given an n x n grid of colored balloons (represented as integers), simulate a chain reaction: find all balloons with at least two neighbors of the same color and remove them, then let the remaining balloons fall down due to gravity. Keep repeating until no more balloons can explode. Return the final grid state.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

The BFS/flood-fill part I got through okay but the gravity step tripped me up more than I expected.

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AI HintsAI Generated

Suggested Approach

Clarify the problem constraints and edge cases first, then propose a simulation approach using BFS/DFS to find and remove clusters of size >= 3, followed by gravity application. Optimize by tracking only affected regions and using a queue to process cascading explosions efficiently.

Pro tip: Demonstrate awareness of performance by discussing time/space complexity and potential optimizations like union-find or in-place gravity, and mention how you'd test with edge cases like no explosions or full-grid cascades.

1. Clarify requirements and constraints

Ask about grid size limits, whether diagonal neighbors count, and if gravity is per-column or global. Confirm that explosions trigger only when a balloon has at least two same-colored neighbors (i.e., cluster size >= 3).

2. Design the explosion detection

Use BFS/DFS to find connected components of same-colored balloons. For each component, if size >= 3, mark for removal. Repeat until no more removals occur.

3. Implement gravity

After each removal round, apply gravity by shifting remaining balloons down in each column, filling empty spaces with zeros (or a sentinel). This can be done in-place with two pointers per column.

4. Optimize and handle cascades

Instead of rescanning the entire grid after each removal, only check neighbors of removed cells for new clusters. Use a queue to process potential explosions efficiently.

5. Analyze complexity and test

Discuss time complexity (worst-case O(n^3) for naive, but can be improved) and space complexity. Walk through edge cases: no explosions, full-grid explosion, and multiple cascades.

Key Points to Mention

Use BFS/DFS to identify connected components of same-colored balloons.
Explosion condition: cluster size >= 3 (since at least two neighbors of same color).
Gravity implementation: per-column compaction, O(n) per column.
Cascading effect: repeat until stable, but optimize by only checking affected areas.
Time complexity: naive O(n^3) per iteration, but can be optimized with union-find or event-driven simulation.
Edge cases: empty grid, no explosions, full-grid explosion, and multiple simultaneous explosions.

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