BulbSwitcher.java

/*
There are n bulbs that are initially off. You first turn on all the bulbs. Then, you turn off every second bulb. On the third round, you toggle every third bulb (turning on if it's off or turning off if it's on). For the nth round, you only toggle the last bulb. Find how many bulbs are on after n rounds.

Example:
Given n = 3. 
At first, the three bulbs are [off, off, off].
After first round, the three bulbs are [on, on, on].
After second round, the three bulbs are [on, off, on].
After third round, the three bulbs are [on, off, off]. 
So you should return 1, because there is only one bulb is on.
*/
// 因为对于某一个灯i, 其约数都是成对的，成对的一开一关，只有当是i的开方的时候才开关了一次。题目问几个灯开着，实际上是问小于等于i的开方的数是多少个。

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