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Early-stage Startup·Software Engineer·Technical Phone Screen·Junior

JuniorRejected
Jul 2026Remote

Summary

Bombed a technical phone screen after a week of prep. The problem itself was embarrassingly simple in hindsight, but I got so deep in my own head about fancy algorithms that I missed the obvious solution entirely.

Questions Asked (1)

Q1

Given a flat array of node objects, each with an id and a children array of ids, find the root node of the tree.

Algorithms & Data Structures
Author's notes

I'd been grinding DFS/BFS problems all week and my brain was stuck in that mode.

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AI HintsAI Generated

Suggested Approach

Clarify the problem constraints and edge cases, then propose an efficient solution using a set to track all child IDs and identify the root as the node whose ID is not in that set. Discuss time and space complexity, and consider alternative approaches like building a parent map or using in-degree counting.

Pro tip: Mention that you can solve it in O(n) time by tracking child IDs in a set, and note that if the tree is guaranteed to have exactly one root, you can stop early once found. This shows attention to efficiency and edge cases.

1. Clarify the problem

Ask about assumptions: Is it a valid tree? Are there multiple roots? Can nodes have duplicate IDs? What should be returned if no root exists?

2. Choose an approach

Decide between using a set to track child IDs, building a parent map, or counting in-degrees. The set approach is simple and efficient.

3. Implement the solution

Iterate through the array to collect all child IDs into a set, then iterate again to find the node whose ID is not in the set.

4. Analyze complexity

State that the time complexity is O(n) and space complexity is O(n) due to the set, where n is the number of nodes.

5. Test with edge cases

Consider cases like a single node, multiple roots, or invalid input, and explain how the solution handles them.

Key Points to Mention

  • Use a set to store all child IDs for O(1) lookups.
  • The root is the only node whose ID is not in the child set.
  • Time complexity: O(n) with two passes; space complexity: O(n).
  • Edge cases: empty array, multiple roots, single node.
  • Alternative: build a parent map or count in-degrees.
  • If the tree is guaranteed to have one root, you can return early.

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