## Problem Simulate a simplified limit order book. You receive a stream of orders: each order is `{id, side, price, quantity}` where `side` is `"buy"` or `"sell"`. - Buy orders are matched against the lowest-priced sell order at or below the buy price. - Sell orders are matched against the highest-priced buy order at or above the sell price. - On price tie, earlier orders have priority (FIFO). - Unmatched remainder stays in the book. After processing all orders, return the remaining orders in the book grouped by side. ```python def process_orders(orders: list[dict]) -> dict: # returns {"buy": [...], "sell": [...]} pass ``` **Example:** ``` Input: [{id:1, side:"sell", price:100, qty:5}, {id:2, side:"buy", price:102, qty:3}, {id:3, side:"buy", price:99, qty:2}] Order 2 matches order 1 for qty=3 @ price=100. Sell order 1 has 2 remaining. Order 3 (buy @99) cannot match sell @100. Output: {"buy": [{id:3,...}], "sell": [{id:1, qty:2,...}]} ``` ## Follow-ups 1. Which data structure gives you O(log n) insert and O(1) best-price lookup for each side? 2. How would you handle partial fills and order cancellations? 3. At 1 million orders/second, where are the bottlenecks in your implementation? 4. How would you persist the order book state across system restarts?

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Optiver Graduate SWE 2025 OA Q2: Order Book Simulation with Price-Time Priority

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Problem Simulate a simplified limit order book. You receive a stream of orders: each order is {id, side, price, quantity} where side is "buy" or "sell". Buy orders are matched against the lowest-priced sell order at or below the buy price. Sell orders are matched against the highest-priced buy order at or above the sell price. On price tie, earlier orders have priority (FIFO). Unmatched remainder stays in the book. After processing all orders, return the remaining orders in the book grouped by s…

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This is a reported interview question from a optiver interview for a swe role during the oa round.

It covers the following topics: Coding, Queue, Oa .

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Optiver Graduate SWE 2025 OA Q2: Order Book Simulation with Price-Time Priority

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