poj1595

素数筛

#include <iostream>
#include <cstdlib>
#include <cstdio>
#include <cstring>
#include <cmath>
using namespace std;

#define maxn 1005
#define maxm 1005

bool is[maxn];
int prm[maxm];
int n, m;
int sum[maxn];

int getprm(int n)
{
    int i, j, k = 0;
    int s, e = (int) (sqrt(0.0 + n) + 1);
    memset(is, 1, sizeof(is));
    prm[k++] = 1;
    prm[k++] = 2;
    is[0] = 0;
    for (i = 4; i < n; i += 2)
        is[i] = 0;
    for (i = 3; i < e; i += 2)
        if (is[i])
        {
            prm[k++] = i;
            for (s = i * 2, j = i * i; j < n; j += s)
                is[j] = 0;
        }
    for (; i < n; i += 2)
        if (is[i])
            prm[k++] = i;
    return k;
}

void work(int n, int m)
{
    int a, b;
    if (n & 1)
    {
        m--;
        a = n / 2 - m;
        b = n / 2 + m;
    }else
    {
        a = n / 2 - m;
        b = n / 2 + m - 1;
    }
    if (a < 0)
        a = 0;
    if (b >= n)
        b = n - 1;
    for (int i = a; i <= b; i++)
        printf(" %d", prm[i]);
    puts("\n");
}

int main()
{
//    freopen("t.txt", "r", stdin);
    getprm(1001);
    memset(sum, 0, sizeof(sum));
    for (int i = 1; i <= 1000; i++)
        if (is[i])
            sum[i] = sum[i - 1] + 1;
        else
            sum[i] = sum[i - 1];
    while (scanf("%d%d", &n, &m) != EOF)
    {
        printf("%d %d:", n, m);
        work(sum[n], m);    
    }
    return 0;
}