There are $$250$$ lamps in a room, numbered with the natural numbers $$1$$, $$2$$, $$\cdots $$, $$250$$, and each light has a switch. At the beginning, all the lights are off. There are $$250$$ individuals who enter the room in turn. The $$1st$$ person enters the room, presses all the switch of the light , and then he leaves. The $$2nd$$ person enters the room, presses the switch for the light numbered in multiples of $$2$$, then he leaves. So on until the $$250th$$ person enters the room and presses the switch for the light number in multiples of $$250$$. After the $$250th$$ person left the room, how many lights were still on in the room? 
Overseas Competition->Knowledge Point->Number Theory Modules->Perfect Square Numbers