From 311bc9f38588e1e9db6d0861612fd1809f7decdb Mon Sep 17 00:00:00 2001
From: Ren Baoshuo <i@baoshuo.ren>
Date: Mon, 23 Aug 2021 16:15:42 +0800
Subject: [PATCH] =?UTF-8?q?868.=20=E7=AD=9B=E8=B4=A8=E6=95=B0?=
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https://www.acwing.com/problem/content/submission/code_detail/7326048/
---
 AcWing/868/868.cpp | 23 +++++++++--------------
 1 file changed, 9 insertions(+), 14 deletions(-)

diff --git a/AcWing/868/868.cpp b/AcWing/868/868.cpp
index feb9698f..6258c0cc 100644
--- a/AcWing/868/868.cpp
+++ b/AcWing/868/868.cpp
@@ -2,25 +2,20 @@
 
 using namespace std;
 
-bool isprime[1000005];
-int n, ans;
+int n, p, primes[1000005];
+bool not_prime[1000005];
 
 int main() {
     cin >> n;
-    for (int i = 0; i <= n; i++) {
-        isprime[i] = true;
-    }
-    isprime[0] = isprime[1] = false;
     for (int i = 2; i <= n; i++) {
-        if (isprime[i] && 1ll * i * i <= n) {
-            for (int j = i * i; j <= n; j += i) {
-                isprime[j] = false;
-            }
+        if (!not_prime[i]) {
+            primes[p++] = i;
+        }
+        for (int j = 0; primes[j] * i <= n; j++) {
+            not_prime[primes[j] * i] = true;
+            if (i % primes[j] == 0) break;
         }
     }
-    for (int i = 1; i <= n; i++) {
-        ans += isprime[i];
-    }
-    cout << ans;
+    cout << p << endl;
     return 0;
 }