LightOJ - 1259数论基础

题目链接

用素数筛选法选素数，记得用bool类型的来赛选否则空间不够用

#include<cstdio>
#include<cstring>
#include<cmath>
typedef long long int ll;
const int mx = 1e7;
int prime[mx/10+5];
bool vis[mx+5];
int len = 0;
void init(){
	vis[1] = 1;
	for(int i = 2; i <= mx; i++)
		if(!vis[i]) {
			prime[len++] = i;
			for(int j = 2*i; j <= mx; j += i)
				vis[j] = 1;
		}
}
int main(){
	int t;
	init();
	scanf("%d",&t);
	int n;
	for(int casei = 1; casei <= t; casei++){
		int ans = 0;
		scanf("%d",&n);
		for(int i = 0; i < len && prime[i] <= n/2; i++)
			if(!vis[n-prime[i]])
				ans++;
		printf("Case %d: %d\n",casei,ans);
	}
	return 0;
}