筛完质数就差不多做完了

#include <iostream>
#include <stack>
#include <cmath>
#include <vector>
#include <string.h>
#include <queue>
#include <stdio.h>
#include <iomanip>
#include <cstdio>
#include <algorithm>
#define int long long
using namespace std;
const int N = 1e5 + 10;
const int INF = 0x3f3f3f3f;
int s;
bool prime[N];
void find_prime(int x)
{
    prime[1] = 1;
	for(int i = 2; i * i <= x; i++)
	{
		if(prime[i] == 0)
		{
			for(int j = i * i; j <= x; j += i)
			{
				prime[j] = 1;
			}
		}
	}
}
signed main()
{
    int s;
    cin >> s;
    find_prime(s);
    if(s % 2 == 1)
    {
        cout << 2 * (s - 2) << endl;
        return 0;
    }
    int maxx = -INF;
    for(int i = 1; i <= s; i++)
    {
        if(prime[i] == 0)
        {
            if(prime[s - i] == 0)
            {
                maxx = max(maxx, i * (s - i));
            }
        }
    }
    cout << maxx << endl;
	return 0;
}

#include<iostream>
using namespace std;
int n,maxx=-1;
bool check(int n){//判断质数
	for(int i=2;i*i<=n;i++){
		if(n%i==0)return false;
	}
	return true;
}
int main(){
	cin>>n;
	for(int i=2;i<=n/2;i++){
		int a=n-i;
		if(check(i)&&check(a)&&i*a>maxx)maxx=i*a;
	}
	cout<<maxx;
	return 0;
}

#include<bits/stdc++.h> using namespace std; bool check(int k){ int i; for(i=3;ii<=k;i++){ if(k%i==0){ return false; } } return true; } int main(){ int a=1,b,i,j,max=0,s; cin>>s; for(i=3;i<=s;i+=2){ b=s-i; a=1; if(check(b)&&check(i)){ a=bi; if(a>max){ max=a; } } } cout<<max; return 0; }//暴力枚举