注意不要重複計數就ok了，奉上java滿分程式碼

import java.util.Scanner;

public class Main {
    public static void main(String[] args){
        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        int[] numArr = new int[n];
        for(int i = 0; i < n; i++){
            numArr[i] = scanner.nextInt();
        }
        scanner.close();

        int count = 0;
        for(int i = 0; i < n; i++){
            int numI = numArr[i];
            for(int j = i + 1; j < n; j++){
                if(numArr[j] + numI == 0){
                    count++;
                    break;
                }
            }
        }

        System.out.println(count);
    }
}