[leetcode] 319.Bulb Switcher

[leetcode] 319.Bulb Switcher

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.

这道题很有意思，其实就是把前20次模拟出来之后就可以找到规律，其实是求1到n 中的平方数的个数。

class Solution {public:    int bulbSwitch(int n) {        if (n < 2) return n;        return (int)sqrt(n);    }};