Ziphq·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Jun 2026

Summary

Ziphq software engineer interview with a grid pathfinding problem. The question sounds straightforward but there are enough edge cases to trip you up if you're not careful about how you handle termination.

Questions Asked (1)

Given an infinite 2D grid with a start cell, an end cell, and a set of barrier cells, find and return any one shortest path (as a sequence of coordinates) between start and end using four-directional movement. If no path exists, return an appropriate indicator rather than running indefinitely.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

The 'infinite grid' part is what got me thinking too hard at first.

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AI HintsAI Generated

Suggested Approach

Use BFS to find the shortest path in an unweighted grid, treating the infinite grid as implicitly bounded by the start, end, and barriers. Track visited cells and parent pointers to reconstruct the path, and return null or a sentinel if the queue exhausts without reaching the end.

Pro tip: Mention that BFS is optimal for unweighted grids and discuss how to handle the infinite grid by only exploring cells within the bounding box of start, end, and barriers (expanded by one). Also, note that if the end is unreachable, BFS will naturally terminate once all reachable cells are explored.

1. Clarify assumptions and constraints

Confirm movement is 4-directional, barriers are impassable, and start/end are not barriers. Discuss whether the grid is truly infinite and how to bound the search space.

2. Choose BFS for shortest path

Explain why BFS guarantees the shortest path in an unweighted graph. Mention that DFS or A* could be alternatives but BFS is simplest and optimal here.

3. Implement BFS with visited set and parent tracking

Use a queue to explore level by level, a set to avoid revisiting cells, and a map to store each cell's parent for path reconstruction.

4. Handle termination and path reconstruction

If the queue empties without reaching the end, return null. Otherwise, backtrack from end to start using parent pointers to build the path.

5. Analyze complexity and edge cases

Discuss time and space complexity in terms of reachable cells. Mention edge cases: start equals end, no path, barriers surrounding start or end.

Key Points to Mention

BFS guarantees shortest path in unweighted graphs
Use a visited set to avoid cycles and infinite loops
Parent map for path reconstruction
Bound the search space using the bounding box of start, end, and barriers
Return null or a sentinel value if no path exists
Time and space complexity: O(N) where N is number of reachable cells

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