牛客_素数对

素数对
https://www.nowcoder.com/practice/c96d6acc025541ffb79c579688f8d003

import java.util.Scanner;

public class Main {
    public static void main(String[] args){
        Scanner in = new Scanner(System.in);

        while(in.hasNext()){
            solution(in);
        }
    }

    /**
     * 数学法
     * @param in
     */
    private static void solution(Scanner in){
        int n = in.nextInt();

        int count = 0;
        for(int num=2,remain=n-num; num<=remain; num++,remain--){
            // 素数对
            if(isPrime(num) && isPrime(remain)){
                count++;
            }
        }

        System.out.println(count);
    }

    /**
     * 是否素数
     * @param num
     * @return
     */
    private static boolean isPrime(int num){
        if(num <= 1){
            return false;
        }
        if(num == 2){
            return true;
        }

        for(int i=2; i<=Math.sqrt(num); i++){
            if(num%i == 0){
                return false;
            }
        }

        return true;
    }
}