F - Goldbach`s Conjecture

 1 #include <bits/stdc++.h>
 2 using namespace std;
 3 #define debug(a) cout<<#a<<":"<<a<<endl;
 4 typedef long long ll;
 5 const int INF=1e9+7;
 6 const ll N=1e7+7;
 7 const int mod=1e9+7;
 8 int T,n;
 9 bool isprime[N];
10 int prime[N/10];
11 int m;
12 void primes(){
13     memset(isprime,false,sizeof(isprime));
14     isprime[1]=true;
15     m=1;
16     for(ll i=2;i<N;i++){
17         if(!isprime[i]){
18             prime[m++] = i;
19             for(ll j=i*i;j<N;j+=i){
20                 isprime[j] = true;
21             }
22         }
23     }
24 }
25 int main(){
26     primes();
27     cin>>T;
28     int a,num;
29     for(int i=1;i<=T;i++){
30         scanf("%d",&a);
31         num=0;
32         for(int j=1;2*prime[j]<=a;j++){
33             if(!isprime[a-prime[j]]){
34                 num++;
35             }
36         }
37         printf("Case %d: %d\n",i,num);
38     }
39     return 0;
40 }