当然也可以用set

#include<bits/stdc++.h>
using namespace std;
long long n,x,tot;
set<int>a;
set<int>::iterator it;
int main(){
	freopen("sumx.in","r",stdin);
	freopen("sumx.out","w",stdout);
	scanf("%d",&n);
	for(int i=1;i<=n;i++){
		int y;
		scanf("%d",&y);
		a.insert(y);
	}
	scanf("%d",&x);
	for(it=a.begin();it!=a.end();it++){
		int hh=x-*it;
		if(a.find(hh)!=a.end())tot++;
	}
	printf("%d",tot/2);//因为会重复找一遍
	return 0;
}