<p>There are <code>n</code> bulbs that are initially off. You first turn on all the bulbs, then you turn off every second bulb.</p> <p>On the third round, you toggle every third bulb (turning on if it's off or turning off if it's on). For the <code>i<sup>th</sup></code> round, you toggle every <code>i</code> bulb. For the <code>n<sup>th</sup></code> round, you only toggle the last bulb.</p> <p>Return <em>the number of bulbs that are on after <code>n</code> rounds</em>.</p> <p> </p> <p><strong>Example 1:</strong></p> <img alt="" src="https://assets.leetcode.com/uploads/2020/11/05/bulb.jpg" style="width: 421px; height: 321px;" /> <pre> <strong>Input:</strong> n = 3 <strong>Output:</strong> 1 <strong>Explanation:</strong> 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.</pre> <p><strong>Example 2:</strong></p> <pre> <strong>Input:</strong> n = 0 <strong>Output:</strong> 0 </pre> <p><strong>Example 3:</strong></p> <pre> <strong>Input:</strong> n = 1 <strong>Output:</strong> 1 </pre> <p> </p> <p><strong>Constraints:</strong></p> <ul> <li><code>0 <= n <= 10<sup>9</sup></code></li> </ul>