AcWing 196. 质数距离（二次筛法 区间筛质数）

#include<bits/stdc++.h>
using namespace std;
#define int long long
const int N=5e6+10;
bool st[N];
int pr[N],cnt;
void init(int x){
    memset(st,0,sizeof st);
    for(int i=2;i<=x;i++){
        if(!st[i]){
            pr[cnt++]=i;
            for(int j=0;pr[j]*i<=x;j++){
                st[pr[j]*i]=1;
                if(i%pr[j]==0)break;
            }
        }
    }
}
signed main(){
    int l,r;
    while(cin>>l>>r){
        init(50005);
        memset(st,0,sizeof st);
        for(int i=0;i<cnt;i++){
            int p=pr[i];
            for(int j=max(2*p,(l+p-1)/p*p);j<=r;j+=p){
                st[j-l]=1;
            }
        }
        cnt=0;
        for(int i=l;i<=r;i++){
            if(!st[i-l]&&i>=2){
                pr[cnt++]=i;
            }
        }
        int maxx=-1,minn=-1;
        if(cnt<2){
            puts("There are no adjacent primes.");
        }
        else{
            for(int i=0;i+1<cnt;i++){
                int d=pr[i+1]-pr[i];
                if(maxx==-1||pr[maxx+1]-pr[maxx]<d)maxx=i;
                if(minn==-1||pr[minn+1]-pr[minn]>d)minn=i;
            }
            cout<<pr[minn]<<","<<pr[minn+1]<<" are closest, "<<pr[maxx]<<","<<pr[maxx+1]<<" are most distant."<<endl;
        }
    }
}