From 444a999efe8299238dbde6d6a4e48a9526376f3c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=E8=BD=A9=E8=BE=95=E9=BE=99=E5=84=BF?= <hyy>
Date: Tue, 12 Apr 2022 10:59:11 +0800
Subject: [PATCH] =?UTF-8?q?204:=E8=AE=A1=E6=95=B0=E8=B4=A8=E6=95=B0?=
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---
 .../java/leetcode/editor/cn/CountPrimes.java  | 76 +++++++++++++++++++
 .../editor/cn/doc/content/CountPrimes.md      | 34 +++++++++
 2 files changed, 110 insertions(+)
 create mode 100644 src/main/java/leetcode/editor/cn/CountPrimes.java
 create mode 100644 src/main/java/leetcode/editor/cn/doc/content/CountPrimes.md

diff --git a/src/main/java/leetcode/editor/cn/CountPrimes.java b/src/main/java/leetcode/editor/cn/CountPrimes.java
new file mode 100644
index 0000000..e16f99d
--- /dev/null
+++ b/src/main/java/leetcode/editor/cn/CountPrimes.java
@@ -0,0 +1,76 @@
+//给定整数 n ，返回 所有小于非负整数 n 的质数的数量 。 
+//
+// 
+//
+// 示例 1： 
+//
+// 
+//输入：n = 10
+//输出：4
+//解释：小于 10 的质数一共有 4 个, 它们是 2, 3, 5, 7 。
+// 
+//
+// 示例 2： 
+//
+// 
+//输入：n = 0
+//输出：0
+// 
+//
+// 示例 3： 
+//
+// 
+//输入：n = 1
+//输出：0
+// 
+//
+// 
+//
+// 提示： 
+//
+// 
+// 0 <= n <= 5 * 10⁶ 
+// 
+// Related Topics 数组 数学 枚举 数论 👍 869 👎 0
+
+package leetcode.editor.cn;
+
+import java.util.Arrays;
+
+//204:计数质数
+public class CountPrimes {
+    public static void main(String[] args) {
+        Solution solution = new CountPrimes().new Solution();
+        // TO TEST
+        solution.countPrimes(499979);
+    }
+
+    //leetcode submit region begin(Prohibit modification and deletion)
+    class Solution {
+        public int countPrimes(int n) {
+            if (n <= 2) {
+                return 0;
+            }
+            boolean[] nums = new boolean[n + 1];
+            Arrays.fill(nums, true);
+            nums[0] = false;
+            nums[1] = false;
+            int count = 0;
+            int max = (int) Math.sqrt(n);
+            for (int i = 2; i < n; i++) {
+                if (nums[i]) {
+                    count++;
+                    if (i > max) {
+                        continue;
+                    }
+                    for (int j = i; j * i < n; j++) {
+                        nums[j * i] = false;
+                    }
+                }
+            }
+            return count;
+        }
+    }
+//leetcode submit region end(Prohibit modification and deletion)
+
+}
diff --git a/src/main/java/leetcode/editor/cn/doc/content/CountPrimes.md b/src/main/java/leetcode/editor/cn/doc/content/CountPrimes.md
new file mode 100644
index 0000000..a80833d
--- /dev/null
+++ b/src/main/java/leetcode/editor/cn/doc/content/CountPrimes.md
@@ -0,0 +1,34 @@
+<p>给定整数 <code>n</code> ，返回 <em>所有小于非负整数&nbsp;<code>n</code>&nbsp;的质数的数量</em> 。</p>
+
+<p>&nbsp;</p>
+
+<p><strong>示例 1：</strong></p>
+
+<pre>
+<strong>输入：</strong>n = 10
+<strong>输出：</strong>4
+<strong>解释：</strong>小于 10 的质数一共有 4 个, 它们是 2, 3, 5, 7 。
+</pre>
+
+<p><strong>示例 2：</strong></p>
+
+<pre>
+<strong>输入：</strong>n = 0
+<strong>输出：</strong>0
+</pre>
+
+<p><strong>示例 3：</strong></p>
+
+<pre>
+<strong>输入：</strong>n = 1
+<strong>输出</strong>：0
+</pre>
+
+<p>&nbsp;</p>
+
+<p><strong>提示：</strong></p>
+
+<ul>
+	<li><code>0 &lt;= n &lt;= 5 * 10<sup>6</sup></code></li>
+</ul>
+<div><div>Related Topics</div><div><li>数组</li><li>数学</li><li>枚举</li><li>数论</li></div></div><br><div><li>👍 869</li><li>👎 0</li></div>
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