除了用哈希表，还可以优化寻找的速度

#include<cstdio>
#include<algorithm>
using namespace std;
int n,a[100110],x,ans,mid;
int main()
{
	scanf("%d",&n);
	for (int i=1;i<=n;i++)
	{
		scanf("%d",&a[i]);
	}
	scanf("%d",&x);
	sort(a+1,a+n+1);//排序
	for (int i=1;i<n;i++)//遍历a[i] 
	{
		//二分查找x-a[i]
		int left=i+1,right=n;//确定左指针、右指针
		while (left<=right)
		{
			mid=(left+right)/2;
			if (a[mid]>x-a[i])
			{
				right=mid-1;
			}
			else
			{
				left=mid+1;
			}
		}
		if (right!=0&&a[right]+a[i]==x&&left!=i&&right!=i)
		{
			ans++;
		}
	}
	printf("%d",ans);
	return 0; 
}

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;
}