Min_25筛初级应用：求$[1,n]$内质数个数

代码

#include <bits/stdc++.h>
#define rin(i,a,b) for(int i=(a);i<=(b);++i)
#define irin(i,a,b) for(int i=(a);i>=(a);--i)
#define trav(i,a) for(int i=head[(a)];i;i=e[i].nxt)
typedef long long LL;
using std::cin;
using std::cout;
using std::endl;

inline LL read(){
	LL x=0,f=1;char ch=getchar();
	while(ch<'0'||ch>'9'){if(ch=='-')f=-1;ch=getchar();}
	while(ch>='0'&&ch<='9'){x=(x<<3)+(x<<1)+ch-'0';ch=getchar();}
	return x*f;
}

const LL MAXN=10000000005;
const LL MOD=2999999;
LL n,prm[200005],cnt;
LL num[200005],g[200005],tot;
bool vis[200005];
struct Hash_Table{
	int head[MOD+5],nxt[200005],siz;
	LL to[200005],val[200005];
	inline void insert(LL x,LL y){
		++siz;
		nxt[siz]=head[x%MOD];
		to[siz]=y;
		val[siz]=x;
		head[x%MOD]=siz;
	}
	inline LL operator [] (LL x){
		for(register int i=head[x%MOD];i;i=nxt[i])
			if(val[i]==x) return to[i];
	}
}mp;

void pre_process(LL n){
	rin(i,2,n){
		if(!vis[i]) prm[++cnt]=i;
		for(register int j=1;j<=cnt&&i*prm[j]<=n;++j){
			vis[i*prm[j]]=1;
			if(i%prm[j]==0) break;
		}
	}
}

int main(){
	n=read();
	pre_process((LL)(sqrt(n)+0.5));
	LL nxti;
	for(register LL i=1;i<=n;i=nxti){
		nxti=n/(n/i)+1;
		num[++tot]=n/i;
		mp.insert(num[tot],tot);
		g[tot]=num[tot]-1;
	}
	rin(i,1,cnt){
		rin(j,1,tot){
			if(prm[i]*prm[i]>num[j]) break;
			int k=mp[num[j]/prm[i]];
			g[j]-=g[k]-(i-1);
		}
	}
	printf("%lld",g[1]);
	return 0;
}