My code:

public class Solution {
    public int bulbSwitch(int n) {
        return (int) Math.sqrt(n);
    }
}

reference:
https://discuss.leetcode.com/topic/39558/share-my-o-1-solution-with-explanation

没什么意思，数学题。

Anyway, Good luck, Richardo! — 10/13/2016