学步园

题解:

#include<cstdio>
#include<cstring>
using namespace std;

const int MAX = 2000;
int p[MAX], a[MAX], pn;;

void prime ()
{
    int i, j;
    memset(a,0,sizeof(a));
    p[1] = 1; pn = 1;
    for ( i = 2; i < MAX; i++ )
    {
        if ( ! a[i] ) p[++pn] = i;
        for ( j = 2; j <= pn && i*p[j] < MAX && ( p[j] <= a[i] || a[i] == 0 ); j++ )
            a[i*p[j]] = p[j];
    }
}

int bfind ( int l, int r, int num )
{
    int left = l, right = r;
    while ( left < right )
    {
        int mid = ( left + right ) / 2;
        if ( num == p[mid] )
            return mid;
        if ( num < p[mid] )
            right = mid - 1;
        else if ( num > p[mid] ) left = mid + 1;
    }
    if ( num >= p[left] ) return left;
    else return left - 1;
}

int main()
{
    prime();

    int n, c, l, r, m;
    while ( scanf("%d%d",&n,&c) != EOF )
    {
        m = bfind ( 1, pn, n );
        if ( m % 2 )
        {
            l = m / 2 - c + 2;
            r = m / 2 + c;
        }
        else
        {
            l = m / 2 - c + 1;
            r = m / 2 + c;
        }
        if ( l < 1 ) l = 1;
        if ( r > m ) r = m;
        printf("%d %d:",n,c);
        for ( int i = l; i <= r; i++ )
            printf(" %d",p[i]);
        printf("\n\n");
    }
    return 0;
}