我觉得这题好扯啊。。。根本就测试不出面试者的水平。。。一个灯泡最后亮只有可能是因为开关被用了odd number times。 

第i 个灯泡开关会被处罚在第d轮只有当d divides i。 所以灯泡最后亮着只能是有一个odd number of divisors.

所以灯泡ends up on只有当i 是一个square。所以就是count the square numbers.

sqrt gives you the root of the largest square in the range [1,n]. And 1 is the smallest root. So you have the roots from 1 to sqrt, that's sqrt many roots. Which correspond to the sqrt many squares.