题面

T组询问，每组询问是一个偶数n

验证哥德巴赫猜想

回答n=a+b

且a，b(a<=b)是质数的方案个数

Input

Input starts with an integer T (≤ 300), denoting the number of test cases.

Each case starts with a line containing an integer n (4 ≤ n ≤ 107, n is even).

Sol

垃圾题，枚举质数就好

# include <bits/stdc++.h>

# define RG register

# define IL inline

# define Fill(a, b) memset(a, b, sizeof(a))

using namespace std;

typedef long long ll;

const int __(1e7 + 10);

IL ll Read(){

	char c = '%'; ll x = 0, z = 1;

	for(; c > '9' || c < '0'; c = getchar()) if(c == '-') z = -1;

	for(; c >= '0' && c <= '9'; c = getchar()) x = x * 10 + c - '0';

	return x * z;

}

int prime[__ / 10], num, n, ans;

bool isprime[__];

IL void Prepare(){

	isprime[1] = 1;

	for(RG int i = 2; i <= __; ++i){

		if(!isprime[i]) prime[++num] = i;

		for(RG int j = 1; j <= num && i * prime[j] <= __; ++j){

			isprime[i * prime[j]] = 1;

			if(!(i % prime[j])) break;

		}

	}

}

int main(RG int argc, RG char *argv[]){

	Prepare();

	for(RG int T = Read(), Case = 1; Case <= T; ++Case){

		n = Read(); ans = 0;

		for(RG int i = 1; i <= num && prime[i] + prime[i] <= n; ++i) ans += !isprime[n - prime[i]];

		printf("Case %d: %d\n", Case, ans);

	}

	return 0;

}