Priority Queue for Elevator

These are the core operations we need for the elevator-simulator application. In the elevator simulator:

• The records are events.
• The keys are the time at which those events will occur.
• We always want to execute the event with the ``smallest time'' (closest to the present).
• Executing an event may result in other future events (elevator arriving at lobby and picking up passengers results in elevator planning to arrive at 5th floor at time t+20 seconds).

One nice implementation of a priority queue is the heap.