题解 | #素数伴侣#

import java.util.Scanner;

// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        // 注意 hasNext 和 hasNextLine 的区别
        // while (in.hasNextInt()) { // 注意 while 处理多个 case
        //     int a = in.nextInt();
        //     int b = in.nextInt();
        //     System.out.println(a + b);
        // }
        int len = in.nextInt();
        int[] arr1 = new int[len];
        int[] arr2 = new int[len];
        int index1 = 0;
        int index2 = 0;
        for (int i = 0; i < len; i++) {
            int n = in.nextInt();
            if (n % 2 == 1) {
                arr1[index1] = n;
                index1++;
            } else {
                arr2[index2] = n;
                index2++;
            }
        }
        int[] matched = new int[index2];

        int count = 0;
        for (int i = 0; i < index1; i++) {
            boolean[] matchArr2 = new boolean[index2];
            if (find(arr1[i], matched, arr2, matchArr2)) {
                count++;
            }
        }
        System.out.print(count);

    }
    private static boolean find(int n, int[] matched, int[] arr2, boolean[] matchArr2) {
        boolean find = false;
        for (int i = 0; i < arr2.length; i++) {
            boolean is = isPrime(n + arr2[i]);

            if (arr2[i] == 0) {
                break;
            }
            if (isPrime(n + arr2[i]) && !matchArr2[i]) {
                matchArr2[i] = true;

                if (matched[i] == 0 || find(matched[i], matched, arr2, matchArr2)) {
                    matched[i] = n;
                    find = true;
                    break;
                } 
            }
        }
        return find;

    }
    private static boolean isPrime(int n) {
        boolean b = true;
        for (int i = 2; i <= Math.sqrt(n); i++) {
            if (n % i == 0) {
                b = false;
                break;
            }
        }
        return b;
    }
}