[poj] 2689 Prime Distance

#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;
}