LightOJ - 1259(素数筛）

链接：https://vjudge.net/contest/288256#problem/F

#include<iostream>
#include<algorithm>
#include<cstdio>
#include<cstring>
using namespace std;
const int sz = 1e7+1;

int prime[sz+1],sr[sz+1],num;

void solve1(){
	for(int i = 1;i<=sz;++i){
		prime[i] = i;
	}
	for(long long i = 2;i<=sz;++i){
		if(prime[i] == i){
			sr[num++] = i; 
			for(long long j = i*i;j<=sz;j+=i){
				prime[j] = 0;
			}
		}
	}
}

int main(){
	num = 0;
	solve1();
	int t,n,ans,id = 0;
	scanf("%d",&t);
	while(t--){
		scanf("%d",&n);
		ans = 0;
		for(int i = 0;sr[i] <= n/2;++i){
			if(prime[n-sr[i]] == (n-sr[i])) ++ans;
		}
		cout<<"Case "<<++id<<": "<<ans<<endl;
	}
	return 0;
}