Got this question from Wish. Anyone knows how to deal with incoming stream of trade orders in a system that can't sort. Is it using PriorityQueue?

Implement a trading system which has the following components:

1. buy(num_of_product, price)
   Called by buyer to buy certain number of products with a maximum price
   num_of_product (INTEGER): number of products to buy
   price (FLOAT): maximal price/unit the buyer is willing to pay
   Return (INTEGER): number of products can be bought

2. sell(num_of_product, price)
   Called by seller to sell certain number of products with a minimum price
   num_of_product (INTEGER): number of products to sell
   price (FLOAT): minimal price/unit the seller is willing to sell
   Return (INTEGER): number of products can be sold

Examples:

system = TradingSystem()
system.sell(50, 1.5)
return: 0

system.sell(20, 1.4)
return: 0

system.buy(60, 1.51)
return: 60

system.buy(20, 1.5)
return: 10

system.sell(20, 0.7)
return: 10

system.buy(100, 0.6)
return: 0