to_be_deleted[classno] = h;
Optical-based smoke alarms use light instead. They are slightly better at detecting the large smoke particles created by slow, smouldering fires. When such particles enter a chamber in the device, they scatter light from a small light source, which is then picked up by a photoelectric sensor.
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《围城》人物分析：知识分子的时代困局
* - this.arr：存储每日股票价格（按调用next的顺序）