素数的个数（挑战程序设计竞赛）

给定整数 n, 请问 n 以内有多少个素数?

输入

• 输入为一个整数 n
• 1≤n≤106

输出

• 输出一个整数表示 n 以内有多少个素数

样例 1

输入

1000000

输出

78498

#include<bits/stdc++.h>
using namespace std;
int Prime[(int)1e6+5];

void pre_prime(int x)
{
    Prime[1]=1;
    for(int i=2;i<=sqrt(x);i++)
    {
        if(Prime[i]==0)
            for(int j=i*i;j<=x;j+=i)
                Prime[j]=1;
    }
}

int main()
{
    int n;
    while(cin>>n)
    {
        pre_prime(n);
        int count=0;
        for(int i=1;i<=n;i++)
            if(Prime[i]==0)
                count++;
        cout<<count<<endl;
    }
}

参考：

https://blog.youkuaiyun.com/jinyj1/article/details/108989951?ops_request_misc=%257B%2522request%255Fid%2522%253A%2522164536372916780271951535%2522%252C%2522scm%2522%253A%252220140713.130102334.pc%255Fall.%2522%257D&request_id=164536372916780271951535&biz_id=0&utm_medium=distribute.pc_search_result.none-task-blog-2~all~first_rank_ecpm_v1~rank_v31_ecpm-3-108989951.pc_search_result_cache&utm_term=%E7%B4%A0%E6%95%B0%E7%AD%9B&spm=1018.2226.3001.4187https://blog.youkuaiyun.com/jinyj1/article/details/108989951?ops_request_misc=%257B%2522request%255Fid%2522%253A%2522164536372916780271951535%2522%252C%2522scm%2522%253A%252220140713.130102334.pc%255Fall.%2522%257D&request_id=164536372916780271951535&biz_id=0&utm_medium=distribute.pc_search_result.none-task-blog-2~all~first_rank_ecpm_v1~rank_v31_ecpm-3-108989951.pc_search_result_cache&utm_term=%E7%B4%A0%E6%95%B0%E7%AD%9B&spm=1018.2226.3001.4187