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Serialize and Deserialize Binary Tree
Serialize and Deserialize forces you to pick a format that survives a round trip, and pre-order with null markers is the cleanest pick. The deserialize side is the real test: a shared cursor over your tokens rebuilds the tree in one pass. Get this and you understand why null placeholders are not optional, they are the structure.
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Low-Level Design Interview Questions
Low-level design problems test whether you can model a small API, preserve invariants, and choose storage that matches the required operations. Expect recurring shapes like rate limiters, hit counters, tic-tac-toe, file systems, and autocomplete.
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