Amazon·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

May 2026

Summary

Amazon SWE interview with a datacenter router simulation problem. Pretty involved for a single coding round, lots of edge cases to track and the Haversine formula thrown in for good measure.

Questions Asked (1)

Implement a function that processes a list of datacenter router commands (REGISTER, SET_HEALTHY, DISTANCE, ROUTE) and returns one output string per command, following specific validation rules and a distance-based load balancing algorithm using Haversine distance.

Algorithms & Data StructuresSystem Design

Author's notes

The REGISTER and SET_HEALTHY parts were fine, just bookkeeping.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the exact command formats, validation rules, and expected outputs, then design a data structure to store router information (location, health status) and implement each command handler with proper error checking. For the ROUTE command, compute Haversine distances from the source router to all healthy routers, select the nearest one, and handle ties or no healthy routers appropriately.

Pro tip: Before coding, walk through a few example commands with the interviewer to confirm edge cases like duplicate registrations, invalid coordinates, and tie-breaking in distance calculations. This shows you think about correctness and edge cases upfront, which is highly valued at Amazon.

1. Clarify Requirements and Edge Cases

Ask questions to understand the exact command syntax, validation rules (e.g., valid coordinates, duplicate IDs), and expected output for each command. Confirm how ties in distance should be resolved and what to do when no healthy routers exist.

2. Design Data Structures

Choose appropriate data structures to store router information, such as a hash map for quick lookup by router ID, and possibly a list or set for healthy routers. Consider how to efficiently retrieve all healthy routers for distance calculations.

3. Implement Command Handlers

Write functions for each command: REGISTER (validate and add router), SET_HEALTHY (update health status), DISTANCE (compute Haversine distance between two routers), and ROUTE (find nearest healthy router from source). Ensure each returns the correct output string or error message.

4. Implement Haversine Distance and Load Balancing

Implement the Haversine formula to compute distances between routers. For ROUTE, iterate over healthy routers, compute distances, and select the one with the smallest distance, handling ties and no healthy routers as per requirements.

5. Test and Validate

Test with provided examples and additional edge cases (e.g., invalid commands, duplicate registrations, routers with same coordinates). Ensure outputs match expected format and all validation rules are enforced.

Key Points to Mention

Haversine formula for great-circle distance between two points on a sphere
Validation rules: unique router IDs, valid latitude/longitude ranges, command syntax
Data structures: hash map for O(1) router lookup, list for healthy routers
Tie-breaking strategy for equal distances (e.g., lexicographically smallest router ID)
Error handling for invalid commands, unregistered routers, or no healthy routers
Time complexity: O(N) for ROUTE where N is number of healthy routers, O(1) for other commands

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