319. Bulb Switcher

1. Question

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 i^th round, you toggle every i bulb. For the n^th round, you only toggle the last bulb.

Return the number of bulbs that are on after n rounds.

2. Examples

Example 1:

Input: n = 3
Output: 1
Explanation: At first, the three bulbs are [off, off, off].
After the first round, the three bulbs are [on, on, on].
After the second round, the three bulbs are [on, off, on].
After the third round, the three bulbs are [on, off, off]. 
So you should return 1 because there is only one bulb is on.

Example 2:

Input: n = 0
Output: 0
Example 3:

Input: n = 1
Output: 1

3. Constraints

• 0 <= n <= 10⁹

4. References

来源：力扣（LeetCode） 链接：https://leetcode-cn.com/problems/bulb-switcher 著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

5. Solutions

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