Order Book Matching Simulation

A HackerRank matching-engine simulation: process a stream of buy/sell limit orders against an order book with price-time priority and return the sum of all executed transaction prices. Naturally solved with two heaps. Requirements Market participants submit buy and sell orders at various prices, forming an order book. When a new buy order at price p arrives: If a sell order exists at price ≤ p, a transaction occurs at that sell order's price; both orders are removed. If multiple sells qualify, the one with the better (lower) price takes priority. Otherwise the buy order is added to the book. A new sell order is symmetric: it matches the highest viable buy (price ≥ the sell price), trading at that buy's price; otherwise it's added. Orders are encoded as a 2-element integer array: first element +1 (buy) or −1 (sell), second element the price. Given a sequence of orders, return the sum of prices across all transactions that occurred. Notes Maintain a max-heap of resting buys and a min-heap of resting sells; on each incoming order, check the opposite book's best price for a match. Each order matches at most once here (one share per order) — both matched orders are removed on a fill. Edge cases: equal prices (a buy at exactly a resting sell's price still trades), and an empty opposite book (order rests). Preparation Implement the two-heap matching engine and dry-run a mixed buy/sell stream by hand to confirm the price-priority and trade-price rules. Practice the symmetric sell-side logic so it mirrors the buy side exactly.