题目描述：

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.

题目大意：

有n盏初始处于关闭状态的灯泡。你首先打开所有的灯泡。然后，熄灭所有序号为2的倍数的灯泡。第三轮，切换所有序号为3的倍数的灯泡（开着的就关掉，关着的就打开）。第n轮，你只切换最后一只灯泡。计算n轮之后还有几盏灯泡亮着。

测试用例见题目描述。

解题思路：

数学题，答案等于int(math.sqrt(n))

对于第i栈灯泡，当i的因子个数为奇数时，最终会保持点亮状态，例如9的因子为1，3，9
而当i的因子个数为偶数时，最终会保持熄灭状态，例如8的因子为：1，2，4，8
当且仅当i为完全平方数时，其因子个数为奇数

为什么只有完全平方数的因子个数为奇数呢？

因为除了完全平方数，其余数字的因子都是成对出现的，而完全平方数的平方根只会统计一次。

前10盏灯泡的开闭过程如下所示：

0 0 0 0 0 0 0 0 0 0    0
1 1 1 1 1 1 1 1 1 1    1
1 0 1 0 1 0 1 0 1 0    2
1 0 0 0 1 1 1 0 0 0    3
1 0 0 1 1 1 1 1 0 0    4
1 0 0 1 0 1 1 1 0 1    5
1 0 0 1 0 0 1 1 0 1    6
1 0 0 1 0 0 0 1 0 1    7
1 0 0 1 0 0 0 0 0 1    8
1 0 0 1 0 0 0 0 1 1    9
1 0 0 1 0 0 0 0 1 0    10

Python代码：

class Solution(object):
    def bulbSwitch(self, n):
        """
        :type n: int
        :rtype: int
        """
        return int(math.sqrt(n))

 

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