解法：抽掉的两个数字之和为2倍的平均数，那么判断一下2倍的平均数是不是整数。然后在搞一个哈希表存取过的值即可。

package com.sky;

import java.util.*;

public class Test1 {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int T = scanner.nextInt();

        while (T-- > 0) {
            int n = scanner.nextInt();
            long[] a = new long[n];
            long sum = 0;

            for (int i = 0; i < n; i++) {
                a[i] = scanner.nextLong();
                sum += a[i];
            }

            if (2 * sum % n != 0) {
                System.out.println(0);
                continue;
            }

            long target = 2 * sum / n;
            Map<Long, Integer> countMap = new HashMap<>();
            long result = 0;

            for (int i = 0; i < n; i++) {
                long complement = target - a[i];
                if (countMap.containsKey(complement)) {
                    result += countMap.get(complement);
                }
                countMap.put(a[i], countMap.getOrDefault(a[i], 0) + 1);
            }

            System.out.println(result);
        }
    }
}