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Tradedesk·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
May 2026

Summary

Phone screen for a software engineer role at Tradedesk. The coding problem was a 3Sum variant with a longer problem statement, and I kind of tripped over myself by not reading it carefully enough before jumping in.

Questions Asked (1)

Q1

Given an array of integers, find all unique triplets that satisfy a specific set of conditions (a variant of the classic 3Sum problem with additional constraints described in a longer problem statement).

Algorithms & Data Structures
Author's notes

Skimmed the problem and missed some of the conditions buried in the description.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the exact conditions and constraints of the problem, then outline a solution that sorts the array and uses a two-pointer technique to find triplets efficiently. Discuss how to handle duplicates and any additional constraints, and analyze the time and space complexity.

Pro tip: Demonstrate awareness of edge cases and trade-offs: mention that sorting enables efficient duplicate skipping and two-pointer traversal, but if the array cannot be modified, consider alternative approaches like hashing. Also, proactively discuss how your solution scales with large inputs.

1. Clarify the problem

Ask questions to confirm the exact conditions for a valid triplet, input size limits, whether the array can be modified, and if the output should be sorted or unique.

2. Choose an approach

Propose sorting the array and using a two-pointer technique for each fixed element to find pairs that satisfy the condition, ensuring O(n^2) time complexity.

3. Handle duplicates and constraints

Explain how to skip duplicate elements to avoid duplicate triplets, and adapt the approach if there are additional constraints (e.g., sum within a range, no sorting allowed).

4. Analyze complexity

State the time complexity (typically O(n^2)) and space complexity (O(1) extra space if sorting in place, or O(n) if using additional data structures).

5. Test with examples

Walk through a small example to verify correctness, including edge cases like empty array, fewer than three elements, or all duplicates.

Key Points to Mention

  • Sorting the array to enable efficient two-pointer search and duplicate skipping
  • Two-pointer technique to reduce time complexity from O(n^3) to O(n^2)
  • Handling duplicates by skipping identical elements after finding a valid triplet
  • Time and space complexity analysis
  • Edge cases: empty input, insufficient elements, all zeros, large input size
  • Trade-offs: sorting vs. hashing, in-place vs. extra space

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