#include<bits/stdc++.h>
using namespace std;
int n, x, a[1000010],sum=0;
int main(){
	cin >> n;
	for(int i = 0;i < n; i++){
		cin >> a[i];
	}
	cin >> x;
	sort(a, a + n);
	for(int i = 0;i < n; i++){
		int idx = lower_bound(a, a + n, x - a[i]) - a;
		if(a[idx] == x - a[i]) sum++;
	}
	cout << sum / 2;
	return 0;
}

萌新写法：枚举+二分

#include <bits/stdc++.h>
using namespace std;

int n,m,size = 0,num[1000005];

int check(int x){
    int left = -1,right = n,mid;
    while(left + 1 < right){
        mid = (left + right) / 2;
        if(num[mid] > x){
            right = mid;
        }
        else{
            left = mid;
        }
    }
    if(num[right - 1] == x){
        return 1;
    }
    return -1;
}


int main(){
    int list;
    cin>>n;
    for(int i = 0 ; i < n ; i++){
        cin>>num[i];
    }
    sort(num,num+n);
    cin>>m;
    for(int i = 0 ; i < n ; i++){
        list = check(m - num[i]);
        if(list != -1){
            size++;
        }
    }
    cout<<size/2<<endl;
    return 0;
}