The tricky part was adapting the recursive pattern from returning a number to building a string.
Use a recursive preorder traversal to serialize the tree: for each node, output its value, then if it has children, output '(' followed by serialized left subtree, a comma if both children exist, serialized right subtree, and ')'. This mirrors the given format and handles missing children by omitting parentheses.
Pro tip: Clarify edge cases upfront: how to represent null nodes (e.g., omit them) and ensure the format is unambiguous for deserialization. Mention that this approach is O(n) time and space, and discuss trade-offs like using delimiters for values to avoid parsing issues.
Parse the example '1(2(4,5),3(6))' to deduce rules: node value, then optional parentheses containing left and right subtrees separated by comma if both exist.
Write a function serialize(node) that returns a string. Base case: if node is null, return empty string. Otherwise, start with node value.
If node has at least one child, append '(' + serialize(left) + (if both children: ',' + serialize(right)) + ')'. If only right child, include a comma to indicate missing left? (Clarify with interviewer).
Walk through the example and edge cases (single node, skewed tree) to ensure output matches format and is unambiguous.
Mention alternative approaches (e.g., level-order with null markers) and why this recursive method is efficient and matches the problem's coordination style.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.