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Hudson River Trading·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
Apr 2026

Summary

HRT coding screen, one problem the whole time. The question looked deceptively simple at first glance but the follow-ups made it clear they wanted something more clever than brute force nested loops.

Questions Asked (1)

Q1

Given two arrays of positive integers, convert each to its decimal string form and find the maximum length of a common prefix shared between any element from the first array and any element from the second array.

Algorithms & Data StructuresTechnical Trade-offs
Author's notes

I started with the naive all-pairs approach and they immediately asked about input size, which was their way of telling me to think harder.

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AI HintsAI Generated

Suggested Approach

Clarify the problem constraints and edge cases, then propose an efficient algorithm using a trie or sorting to find the maximum common prefix length. Discuss trade-offs between time and space complexity, and consider optimizations like early termination.

Pro tip: Mention that converting integers to strings can be avoided by comparing digits arithmetically, which saves memory and time, especially for large arrays.

1. Clarify the problem

Confirm that the common prefix is based on the decimal string representation, and discuss constraints like array sizes and integer ranges.

2. Consider brute force

Acknowledge that comparing all pairs is O(N*M*L) and may be too slow, but it's a starting point for small inputs.

3. Propose efficient approach

Suggest building a trie from one array's strings and querying with the other, or sorting both arrays and using binary search for prefix lengths.

4. Analyze complexity

Compare time and space of trie vs. sorting, noting that trie uses O(total characters) space while sorting is in-place but O(N log N + M log M).

5. Handle edge cases

Discuss empty arrays, single-element arrays, and integers with different digit lengths, ensuring the algorithm returns 0 when no common prefix exists.

Key Points to Mention

  • String conversion overhead and alternatives
  • Trie data structure for prefix matching
  • Sorting and binary search for prefix length
  • Time and space complexity trade-offs
  • Edge cases: empty arrays, no common prefix
  • Early termination when maximum possible prefix is found

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