I recognized the BFS pattern pretty quickly but the -1 separator thing threw me off for a minute.
Clarify the serialization format and edge cases, then design a BFS-based solution that processes the input tokens level by level, using -1 to delimit sibling groups and tracking parent-child relationships. Explain the algorithm, analyze complexity, and discuss potential optimizations or alternative approaches.
Pro tip: Demonstrate thoroughness by discussing how you would handle malformed input or ambiguous separators, and mention that the same BFS pattern applies to many tree problems, showing pattern recognition.
Ask questions to confirm the serialization format, the meaning of -1, and expected output format. Ensure you understand constraints like tree size and value ranges.
Use a queue for BFS. Parse tokens sequentially: when you see a value, create a node and attach it to the current parent; when you see -1, move to the next parent. Track levels by processing the queue level by level.
Consider empty input, single node, multiple consecutive -1s, and trailing -1. Discuss how to handle invalid input gracefully.
State that time complexity is O(N) where N is the number of tokens, and space complexity is O(W) where W is the maximum width of the tree (due to queue).
Walk through a small example to verify correctness, and mention potential pitfalls like misinterpreting -1 as a node value.
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