Nextdoor·Machine Learning Engineer·Technical Phone Screen·Senior

SeniorPrefer not to say

Jun 2026Remote

Summary

Nextdoor ML Engineer interview that was basically a version string sorting problem dressed up with a bunch of edge cases they clearly wanted you to sweat through. One coding round, algorithm-heavy, with a complexity discussion tacked on at the end.

Questions Asked (1)

Given up to 100,000 version-like strings (e.g. '1.0', '01.2.0', '2.0.0-alpha', '1.10.3'), implement a comparator that orders them by splitting on '.', comparing numeric segments as integers, treating missing segments as 0, sorting pre-release tags like '-alpha' and '-beta' before the corresponding release, and ignoring numeric build metadata after '+'. Use the comparator to return the sorted list. Also discuss time/space complexity and edge cases like leading zeros, different segment counts, and non-numeric segments.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

I started with the segment splitting and integer parsing, which felt fine, then the pre-release ordering tripped me up for a bit.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the version format and edge cases, then outline a comparator that parses each version into comparable components (numeric segments, pre-release tags, build metadata). Emphasize a clean implementation using a custom key function or comparator, and discuss complexity and trade-offs.

Pro tip: Mention that you would use a stable sort to preserve the original order of equal versions, and that you would write unit tests for edge cases like leading zeros and missing segments to ensure correctness.

1. Clarify requirements and edge cases

Ask clarifying questions about the version format, such as handling of leading zeros, non-numeric segments, and build metadata. Confirm that missing segments are treated as zero and pre-release tags sort before the release.

2. Design the comparator logic

Outline a parsing strategy: split on '.', separate build metadata after '+', and pre-release after '-'. Compare numeric segments as integers, then compare pre-release tags lexicographically, and finally ignore build metadata.

3. Implement the comparator

Write a function that returns a comparison key or uses a custom comparator. For efficiency, precompute keys for each version to avoid repeated parsing during sorting.

4. Analyze complexity and trade-offs

Discuss time complexity: O(n log n * k) where k is average number of segments, and space complexity O(n * k). Mention that precomputing keys reduces constant factors.

5. Test and validate

Walk through edge cases: leading zeros (e.g., '01.2' vs '1.2'), different segment counts (e.g., '1.0' vs '1.0.0'), pre-release tags (e.g., '1.0-alpha' < '1.0'), and build metadata (e.g., '1.0+build' equals '1.0').

Key Points to Mention

Parsing versions into numeric segments, pre-release tags, and build metadata.
Treating missing segments as zero and comparing numeric segments as integers.
Sorting pre-release tags before the release and comparing them lexicographically.
Ignoring build metadata after '+' for comparison purposes.
Time complexity O(n log n * k) and space complexity O(n * k), with potential optimization by precomputing keys.
Edge cases: leading zeros, different segment counts, non-numeric segments, and stability of sort.

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