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I jumped straight to a recursive solution and it worked but I fumbled explaining the time complexity.
Start by clarifying the input format and assumptions (e.g., whether the array is sorted, if parent references are IDs or direct object references, and if there can be multiple roots). Then propose an efficient O(n) solution using a hash map to index nodes by ID, followed by a second pass to link children to parents. Finally, discuss handling edge cases like missing parents, cycles, and multiple roots.
Pro tip: Mention that you'd validate the data for cycles or missing parents before building the tree, and consider whether to mutate the original objects or create new ones to avoid side effects. This shows you think about robustness and production concerns.
Ask about input size, whether parent references are IDs or object pointers, if there can be multiple roots, and if the array is sorted. Confirm the expected output format (e.g., nested objects with children arrays).
Propose a two-pass approach: first, create a map from ID to node (and initialize children arrays); second, iterate again to attach each node to its parent's children list. Identify root nodes (those with null/undefined parent).
Discuss what to do if a parent ID doesn't exist (e.g., treat as root or throw error), if there are cycles, or if multiple roots exist. Decide whether to return a single root or an array of roots.
State that the solution is O(n) time and O(n) space. Mention that a single-pass approach is possible if nodes are guaranteed to be in topological order, but the two-pass is more robust.
Walk through a small example to verify correctness, including edge cases. Suggest writing unit tests for empty input, single node, multiple roots, and deep nesting.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.