[poj] 2689 Prime Distance

原题

简单的素数筛，要注意mn的初值要大于r-l+1……

#include<cstdio>
#include<algorithm>
#include<cstring>
using namespace std;
typedef long long ll;
bool is[50010],wa[1000010];
ll l,r,x,y,prime[1000010],yxd;

int main()
{
    memset(is,1,sizeof(is));
    for (ll i=2;i<=50000;i++)
    {
	if (is[i])
	{
	    prime[++yxd]=i;
	    for (ll j=i*i;j<=1000000;j+=i)
		is[j]=0;
	}
    }
    while(~scanf("%lld%lld",&l,&r))
    {
	ll pre=0,mn=r-l+2,mx=0,fir=0,lst=0,cnt=0;
	x=0,y=0;
	for (int i=0;i<=mn;i++) wa[i]=1;
	if (1-l>=0) wa[1-l]=0;
	for (ll i=1;i<=yxd && prime[i]*prime[i]<=r;i++)
	{
	    ll j=l/prime[i]*prime[i];
	    for (;j<=r;j+=prime[i])
		if (j>=l && j>prime[i]) wa[j-l]=0;
	}
	for (ll i=l;i<=r;i++)
	    if (wa[i-l])
	    {
		if (pre)
		{
		    if (i-pre+1<mn) x=pre,y=i,mn=i-pre+1;
		    if (i-pre+1>mx) fir=pre,lst=i,mx=i-pre+1;
		}
		pre=i;
		cnt++;
	    }
	if (cnt<2) printf("There are no adjacent primes.\n");
	else printf("%lld,%lld are closest, %lld,%lld are most distant.\n",x,y,fir,lst);
    }
    return 0;
}