poj 2262 Goldbach's Conjecture

 1 #include<cstdio>
 2 #include<algorithm>
 3 #include<iostream>
 4 #include<string>
 5 #include<cstring>
 6 #include<vector>
 7 using namespace std;
 8 #define max 1000000
 9 int prime[max+5];
10 bool vis[max+5];
11 void get_prime(int n){
12     memset(vis,false,sizeof(vis));
13     vis[0]=vis[1]=true;
14     int i=2,index=0;
15     for(;i<=n;i++){
16         if(!vis[i]){
17             prime[index++]=i;
18             //vis[i]=false;
19         }
20         int j=0;
21         for(;j<index&&prime[j]*i<=n;j++){//????
22             vis[prime[j]*i]=true;
23             if(!(i%prime[j]))
24             break;
25         }
26     }
27 }
28 int main(){
29     int n;
30     get_prime(max);
31     while(cin>>n&&n){
32         int i=3,half=n/2;
33         for(;i<=half;i+=2){//等号一定要取到，存在数等于两个相同质数之和的
34             if(!vis[i]&&!vis[n-i]){
35                 break;
36             }
37         }
38         if(i>half)
39         cout<<"Goldbach‘s conjecture is wrong."<<endl;
40         else
41         cout<<n<<" = "<<i<<" + "<<n-i<<endl;
42     }
43     return 0;
44 }