POJ2689:素数区间筛选

#include <cstdio>
#include <cstring>
#include <cmath>
#include <algorithm>
using namespace std;
typedef long long ll;
const int MAXN=1000005;
const int INF=0x7f7f7f7f;
int l,u;
int prime[MAXN],top;
bool isPrime_small[MAXN];
bool isPrime[MAXN];
void segment_sieve()
{
    memset(isPrime_small,true,sizeof(isPrime_small));
    memset(isPrime,true,sizeof(isPrime));
    isPrime_small[1]=false;
    for(ll i=2;i*i<=u;i++) //注意防止i,j溢出,要设为long long
    {
        if(isPrime_small[i])
        {
            for(ll j=i+i;j*j<=u;j+=i)
                isPrime_small[j]=false;
            for(ll j=max(((l+i-1)/i)*i,2*i);j<=u;j+=i)
                isPrime[j-l]=false;
        }
    }
}
void sieve()
{
    for(ll i=l;i<=u;i++)
    {
        if(isPrime[i-l])
            prime[top++]=i;
    }
}
int main()
{
    while(scanf("%d%d",&l,&u)!=EOF)
    {
        if(l==1)    l++;
        top=0;
        segment_sieve();
        sieve();
        if(top<=1)
        {
            printf("There are no adjacent primes.\n");
        }
        else
        {
            int a,b;
            int mn=INF;
            int c,d;
            int mx=0; 
            for(int i=1;i<top;i++)
            {
                if(prime[i]-prime[i-1]<mn)
                {
                    a=prime[i-1];
                    b=prime[i];
                    mn=b-a;
                }
                if(prime[i]-prime[i-1]>mx)
                {
                    c=prime[i-1];
                    d=prime[i];
                    mx=d-c;
                }
            }
            printf("%d,%d are closest, %d,%d are most distant.\n",a,b,c,d);
        }
    }
    return 0;
}