JAVA 代码

import java.util.Scanner;

public class Main{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] arr = new int[1010];
        for(int i = 0;i<n;i++){
            int t = sc.nextInt();
            arr[Math.abs(t)]++;
        }
        int res = 0;
        for(int i =0;i<arr.length;i++)
        {
            if(arr[i] == 2){
                res++;
            }
        }
        System.out.println(res);
    }
}