A building has three elevators, 16-storey level. Write a program to simulate the operation of elevators.
Requirements:
1. At the highest level can only be a downward trend, the lowest level can only be upward, the middle layer can be by any upward and downward;
2. Each layer is only one up and one down button;
3. The system can be used cs, bs, or console program implementation;
4. System input please refer to the actual elevator system, if there is no interface that can be made single-threaded, to facilitate the importation;
5. The system can see the three elevator button on the location and status of each floor;
6. Elevator running every 2 seconds move a layer, if you stay, then stay in a layer of 4 seconds;
7. System Interface refresh every 2 seconds;
8. Elevator operation strategy are free to design;
