An online computer system for trading stocks needs to process orders of the form buy 100 shares at $x each or sell 100 shares at $y each. A buy order for $x can only be processed if there is an existing sell order with price $y such that y x. Likewise, a sell order for $y can only be processed if there is an existing buy order with price $x such that y x. If a buy or sell order is entered but cannot be processed, it must wait for a future order that allows it to be processed. Describe a scheme that allows buy and sell orders to be entered in O(log n) time, independent of whether or not they can be immediately processed.