【數論和基礎數學】質數篩選-蒜頭君的猜想

【數論和基礎數學】質數篩選-蒜頭君的猜想

//
//  main.cpp
//  hahah
//
//  Created by apple on 2019/9/6.
//  Copyright © 2019 apple. All rights reserved.
//

#include<iostream>

using namespace std;
int main(){
    int n;
    scanf("%d",&n);
    int c = n + 1;
    int *is_prime = (int*)malloc(sizeof(int)*c);
    //都是質數
    for(int i = 2; i <= n; i++){
        is_prime[i] = 1;
    }
    //篩選質數
    for(int i = 2; i*i <= n; i++){
        if(is_prime[i]){
            for(int j = i*i; j <= n; j += i){
                is_prime[j] = 0;
            }
        }
    }

    int ans = 0; // answer
    for(int i = 2; i <= (n/2); i++){
        if(is_prime[i]&&is_prime[n-i]){
            ans++;
        }
    }
    printf("%d\n",ans);
    free(is_prime);
    return 0;
}