BulbSwitcher.java

package leetcode;

/**
 * Project Name : Leetcode
 * Package Name : leetcode
 * File Name : BulbSwitcher
 * Creator : Edward
 * Date : Dec, 2017
 * Description : 319. Bulb Switcher
 */
public class BulbSwitcher {
    /**
     * 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.

     time : O(1)
     space : O(1)


     * @param n
     * @return
     */
    public int bulbSwitch(int n) {
        return (int)Math.sqrt(n);
    }
}