欧拉筛线性筛质数

欧拉筛线性筛质数

经典题解

我的乱搞筛法和欧拉线性筛法的速度对比：

模版code

#include <bits/stdc++.h>
using namespace std;
const int maxn = 100000009;
bool a[100000009];
int read()
{
    int x=0,f=1;char ch=getchar();
    while(!isdigit(ch)){if(ch=='-')f=-1;ch=getchar();}
    while(isdigit(ch)){x=x*10+ch-'0';ch=getchar();}
    return x*f;
}
int n, q;
bool isprime(int x)
{
    if (x < 2)
        return false;
    for (int i = 2; i * i <= x; i++)
    {
        if (x % i == 0)
            return false;
    }
    return true;
}
int pr[maxn], x = 0;
int main()
{
    std::ios::sync_with_stdio(0);
    std::cout.tie(0);
    n=read(), q=read();
    a[1]=1;
    for (int i = 2; i <= n; i++)
    {
        if(!a[i])
        pr[++x] = i;
        a[i]=1;
        for (int j = 1; j <= x&&i*pr[j]<=n; j ++)
        {
            a[i*pr[j]]=1;
            if(i%pr[j]==0)
                break;
        }
    }
    for (int i = 1; i <= q; i++)
    {
        int d=read();
        cout<<pr[d]<<'\n';
    }
    return 0;
}

乱搞筛法

#include <bits/stdc++.h>
using namespace std;
const int maxn = 100000009;
bool a[100000009];
int read()
{
    int x=0,f=1;char ch=getchar();
    while(!isdigit(ch)){if(ch=='-')f=-1;ch=getchar();}
    while(isdigit(ch)){x=x*10+ch-'0';ch=getchar();}
    return x*f;
}
int n, q;
bool isprime(int x)
{
    if (x < 2)
        return false;
    for (int i = 2; i * i <= x; i++)
    {
        if (x % i == 0)
            return false;
    }
    return true;
}
int pr[maxn], x = 0;
int main()
{
    std::ios::sync_with_stdio(0);
    std::cout.tie(0);
    n=read(), q=read();
    a[1]=1;
    for (int i = 2; i <= n; i++)
    {
        if (a[i])
            continue;
        pr[++x] = i;
        a[i]=1;
        for (int j = i * 2; j <= n; j += i)
        {
            a[j] = 1;
        }
    }
    for (int i = 1; i <= q; i++)
    {
        int d=read();
        cout<<pr[d]<<'\n';
    }
    return 0;
}