Leetcode ☞ 319. Bulb Switcher ☆ brain teaser

319. Bulb Switcher

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Question

Total Accepted: 11657 Total Submissions: 29503 Difficulty: Medium

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 ith round, you toggle every i bulb. 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.

我的AC：

int bulbSwitch(int n) {    return sqrt(n);}

分析：

看的大神解答....=.= ： http://www.zhihu.com/question/35485418?sort=created  搜“bulb”即可找到

截图如下：

最终就是找n之前有几个完全平方数，直接给n开平方就得到了！