pku 2909 Goldbach's Conjecture

筛法求素数

 

code:

#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#define MAXN 35000
using namespace std;
bool prime[MAXN];
void is_prime()
{
    int i, j;
    for(i=2; i<MAXN; i++)
    {
        if(prime[i]==0)
        for(j=i+i; j<MAXN; j+=i)
            prime[j]=1;
    }

}

int main()
{
    memset(prime, 0, sizeof(prime));
    is_prime();
    int n, m;
    while(scanf("%d", &n), n)
    {
        int count(0);
        m=n/2;
        for(int i=2; i<=m; i++)
        {
            if(prime[i]==0)
            {
                if(prime[n-i]==0)
                count++;
            }
        }
        printf("%d\n", count);
    }
    return 0;
}