loj10200. 「一本通 6.2 练习 3」Goldbach's Conjecture

思路：
　　筛法筛出素数，逐个判断即可。

#include<cstdio>
#include<iostream>
#include<cmath>
using namespace std;
const int maxn = 1000010;
int cnt, a;
int prime[maxn];
int isnprime[maxn];
void findpri(int lim){
    for(int i = 2; i <= lim; ++i){
        if(!isnprime[i])
            prime[++cnt] = i;
        for(int j = 1; j <= cnt; ++j){
            if(i * prime[j] > lim)    break;
            isnprime[i * prime[j]] = 1;
            if(i % prime[j] == 0)        break;
        }
    }
}
int main(void) {
    findpri(maxn - 10);
    while(scanf("%d", &a) && a){
        for(int i = 1; i <= cnt; ++i){
            if(!isnprime[a - prime[i]]){
                printf("%d = %d + %d\n", a, prime[i], a - prime[i]);
                break;
            }
        }
    }
    return 0;
}