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leetcode/0319.Bulb-Switcher/README.md

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+# [319. Bulb Switcher](https://leetcode-cn.com/problems/bulb-switcher/)
+
+
+## 题目
+
+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.
+
+Return the number of bulbs that are on after n rounds.
+
+**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
+
+## 题目大意
+
+初始时有n个灯泡处于关闭状态。第一轮，你将会打开所有灯泡。接下来的第二轮，你将会每两个灯泡关闭一个。
+
+第三轮，你每三个灯泡就切换一个灯泡的开关（即，打开变关闭，关闭变打开）。第i轮，你每i个灯泡就切换一个灯泡的开关。直到第n轮，你只需要切换最后一个灯泡的开关。
+
+找出并返回n轮后有多少个亮着的灯泡。
+
+## 解题思路
+
+- 计算1到n中有奇数个约数的个数
+- 1到n中的某个数x有奇数个约数,也即x是完全平方数
+- 计算1到n中完全平方数的个数sqrt(n)
+
+## 代码
+
+```go
+package leetcode
+
+import "math"
+
+func bulbSwitch(n int) int {
+	return int(math.Sqrt(float64(n)))
+}
+```