还能加点啥油盐酱醋？Slot有large，mid-size，compact之分，搞个继承/多态出来？
class Slot {
public:
int calcFee();
void resetTimer();
private:
int timeStamp;
};
class Manager {
public:
Slot* getFreeSlot();
void reclaimSlot(Slot* s);
private:
list freeSlots;
list occupiedSlot;
};
