vis数组没减L之前跑100ms，减L之后就7s多

#include<stdio.h>
#include<math.h>
#include<string.h>
#include<algorithm>
using namespace std;
const int MAXN=1e6+10;
int L,R,cnt,ans,cnt2;
bool isprime[MAXN];
int primelist[MAXN];
bool vis[MAXN];
inline int read()
{
	int r=0;
	int w=1;
	char ch=getchar();
	while(ch < '0'||ch > '9')
	{
		if(ch == '-')
			w=-1;
		ch=getchar();
	}
	while(ch >= '0'&&ch <= '9')
	{
		r=r*10-'0'+ch;
		ch=getchar();
	}
	return r*w;
}
void judge_prime()
{
	for(int i = 2;i <= 50000; ++i)
	{
		if(isprime[i])
			primelist[++cnt] = i;
		for(int j = 1;j <= cnt&&i*primelist[j] <= sqrt(R); ++j)
		{
			isprime[i*primelist[j]] = false;
			if(i % primelist[j] == 0)
				break;
		}
	}
}
int main()
{
	memset(isprime,true,sizeof(isprime));
	isprime[1] = false;
	L=read();R=read();
	judge_prime();
	for(int i=1;i<=cnt;++i)
		for(int j=2;j<=sqrt(R);++j)
			if(j*primelist[i] <= R&&j*primelist[i] >= L&&!vis[j*primelist[i]-L])
			{
				vis[j*primelist[i]-L]=true;
				cnt2++;
			}
	if(L == 1)
	{
		printf("%d\n",R-L-cnt2);
		exit(0);
	}
	printf("%d\n",R-L-cnt2+1);
	return 0;
}