First, clarify the exact input format and error priority order with the interviewer, then outline a two-phase algorithm: parse and validate the string format, then build the graph while checking structural constraints. Finally, if no errors, reconstruct the tree and print its S-expression, ensuring you handle edge cases like empty input or single node.
Pro tip: Explicitly state the error priority order and confirm it with the interviewer before coding; this shows you understand the importance of requirements clarification and prevents wasted effort on the wrong error.
Ask about the exact format (spaces, parentheses, commas), error priority order, and what to do with empty input or single node. Confirm the definition of a valid S-expression.
Use a regex or manual parser to check each pair matches the expected pattern. If any pair fails, return 'invalid format' immediately.
Iterate through pairs, tracking children per parent, parents per child, and detecting duplicate pairs. Check for >2 children, multiple parents (cycle), and multiple roots in the correct priority order.
If no errors, find the root (node with no parent) and recursively build the S-expression. Handle the case of a single node (no pairs) by printing just the node.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.