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Snapchat·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say
May 2026Remote

Summary

Snapchat backend interview that basically came down to graph traversal. The problem was well-scoped but the follow-up discussion on edge cases is where things got interesting.

Questions Asked (1)

Q1

Given a set of services with directed dependencies and a latency value on each edge, find the minimum latency path between a start service and an end service. Then discuss how you'd handle disconnected graphs, zero-latency edges, cycles, and what the time complexity looks like.

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

I jumped straight to Dijkstra with a priority queue and that part went fine.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the problem: this is a single-source shortest path problem on a directed graph with non-negative edge weights (latencies). Use Dijkstra's algorithm with a priority queue, then systematically address each edge case (disconnected graphs, zero-latency edges, cycles) and analyze time complexity.

Pro tip: Mention that zero-latency edges are fine for Dijkstra, but if negative latencies were possible, you'd need Bellman-Ford; also note that cycles don't break Dijkstra as long as weights are non-negative, but you should detect and handle them if they represent invalid configurations.

1. Clarify the problem and constraints

Confirm that latencies are non-negative, the graph is directed, and we need the minimum total latency from a single start to a single end. Ask if the graph is static or dynamic, and if there are any constraints on graph size.

2. Choose the algorithm

Select Dijkstra's algorithm with a min-heap (priority queue) because it efficiently handles non-negative weights and finds the shortest path from one source to all nodes, from which we extract the target.

3. Handle edge cases

For disconnected graphs, return infinity or indicate no path. Zero-latency edges are allowed and do not affect correctness. Cycles are naturally handled by Dijkstra as long as weights are non-negative; if negative cycles exist, use Bellman-Ford and detect them.

4. Analyze time and space complexity

With a binary heap, Dijkstra runs in O((V + E) log V) time and O(V) space. Mention that using a Fibonacci heap improves to O(E + V log V), but is rarely practical.

5. Discuss trade-offs and alternatives

Compare with Bellman-Ford (O(VE)) for negative weights, BFS for unweighted graphs, and A* if a heuristic is available. Also mention that for very large graphs, bidirectional Dijkstra or contraction hierarchies can be faster.

Key Points to Mention

  • Dijkstra's algorithm is optimal for non-negative weights; use a priority queue for efficiency.
  • Disconnected graphs: if the end service is unreachable, return a sentinel value (e.g., infinity) or raise an error.
  • Zero-latency edges are valid and do not violate Dijkstra's assumptions; they can be processed in any order.
  • Cycles are fine with non-negative weights, but if negative cycles exist, use Bellman-Ford and detect them.
  • Time complexity: O((V + E) log V) with a binary heap; space complexity O(V).
  • Alternatives: Bellman-Ford for negative weights, BFS for unweighted, A* for heuristic-guided search.

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