poj 2689 Prime Distance (两次筛法)

素数筛法就像这样

void calprime(){
    long long i,j;
    memset(vis,0,sizeof(vis));
    priLen = 0;
    for(i = 2; i <= M; i++) 
        if(!vis[i]){
            prime[priLen++] = i;
        for(j = i*i; j <=M; j+=i)
            vis[j] = 1;
    }
}

而双次筛法可以判断一个区间，假设区间为[k,m]只要先用筛法计算出2到sqrt(m)间的素数，然后用同方法计算k到m区间的就可以了

ac——code

#include <iostream>
using namespace std;
#define M 46342
#define _mAX 1000004
long long prime[M];
int priLen;
int vis[M];
int vis2[_mAX];
int prime2[_mAX];
int priLen2;
int nvis[_mAX];
void calprime(){
    long long i,j;
    memset(vis,0,sizeof(vis));
    priLen = 0;
    for(i = 2; i <= M; i++) 
        if(!vis[i]){
            prime[priLen++] = i;
        for(j = i*i; j <=M; j+=i)
            vis[j] = 1;
    }
}
void duoPrime(int l,int r){
    int base,i,t;
    unsigned int j;
    priLen2 = 0;
    if(l < 2)
        l = 2;
    base = l;
    memset(vis2,0,sizeof(vis2));
    memset(prime2,0,sizeof(prime2));
    for(i = 0; i < priLen && prime[i]*prime[i] <= r ; i++){
        if(prime[i]*prime[i] >= l)
            j = prime[i];
        else if(l / prime[i]*prime[i] == l)
            j = l / prime[i];
        else j = (l/prime[i]+1);
        for(j = j*prime[i];j <=r; j+=prime[i]){
            vis2[j-base]=1;
        }
    }
}
void find(int l,int r){
    int _min,_max,_pre,t,s1,s2,s3,s4;
    unsigned int i;
    _min = INT_MAX;
    _max = -1;
    _pre = -1;
    if(l < 2)
        l =2;
    for(i = l; i <= r;i++ ){
        if(vis2[i - l] == 0){
            if(_pre == -1){
                _pre = i;
                continue;
            }
            else {
                t = i - _pre;
                if(t > _max){
                    _max = t;
                    s1 = _pre;
                    s2 = i;
                }
                if(t < _min){
                    _min = t;
                    s3 = _pre;
                    s4 = i;
                }
                _pre = i;
            }
        }
    }
    if(_min == INT_MAX)
        puts("There are no adjacent primes.");
    else printf("%d,%d are closest, %d,%d are most distant.\n",s3,s4,s1,s2);
}
int main(){
    int l,u;
    calprime();
    while(scanf("%d%d",&l,&u)!=EOF){
        duoPrime(l,u);
        find(l,u);
    }
    return 0;
}