class Crawler {
优化：如果某轮没有发生交换，说明已有序，提前结束
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Гангстер одним ударом расправился с туристом в Таиланде и попал на видео18:08
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.