POJ2262

首先生成1000000以内的素数表，采用素数筛选法http://www.cnblogs.com/bournet/p/3972023.html。下面就很简单了。

#include <iostream>
#include <math.h>
using namespace std;

int main()
{
    int N = 1000000;
    bool prime[N+1];
    for (int i = 0; i < N; i++)
    {
        if (i%2 == 0 && i != 2)
            prime[i] = false;
        else
            prime[i] = true;
    }

    for (int i = 3; i <= sqrt(N); i += 2)
    {
        if (prime[i])
            for (int j = i+i; j <= N; j += i)
                prime[j] = false;
    }

    int n;
    while (cin >> n)
    {
        if (n == 0)
            return 0;
        for (int i = 2; i <= n/2; i++)
        {
            if (prime[i])
            {
                if (prime[n-i])
                {
                    cout << n << " = " << i << " + " << n-i << endl;
                    break;
                }
            }
            if (i == n/2)
            {
                cout << "Goldbach's conjecture is wrong." << endl;
            }
        }
    }

    return 0;
}