对称轴，刚开始写的时候直接按顺序加的，这就导致当出现两个以上y相等的时候会NO。后来我有把y相等全部拿出来，第一个和最后一个相加除二，A了。



#include<iostream>
#include<vector>
#include<set>
#include<algorithm>
using namespace std;

struct Point{
    double x, y;
};

set<double> if_count;
vector<Point> all_p;

bool cmp_y(Point a, Point b) {
    if(a.y != b.y)
        return a.y < b.y;
    return a.x < b.x;
}

void judge_count(int star, int en) {
    if(star <= en) {
        judge_count(star + 1, en - 1);
        if_count.insert((all_p[star].x + all_p[en].x) / 2);
    }
}

int main() {
    int T;
    cin >> T;
    while(T--) {
        int N, beg = 0, last = 0;
        cin >> N;
        while(N--) {
            Point p;
            cin >> p.x >> p.y;
            all_p.push_back(p);
        }
        sort(all_p.begin(), all_p.end(), cmp_y);
        for(int i = 1; i < all_p.size(); i++) {
            if(all_p[i].y == all_p[i - 1].y)
                last++;
            else {
                judge_count(beg, last);
                beg = last = i;
            }
        }
        judge_count(beg, last);
        if_count.size() == 1 ? cout << "YES" << endl : cout << "NO" << endl;
        if_count.clear();
        all_p.clear();
    }
    return 0;
}