排序+二分，分别找到中间数的最左边的位置和最右边的位置

#include <iostream>
#include <algorithm>

using namespace std;
const int N = 1010;
int q[N],n;

int find_left(int x){
    int l = 0,r = n - 1;
    while(l < r){
        int mid = l + r >> 1;
        if(q[mid] >= x) r = mid;
        else l = mid + 1;
    }
    return l;
}

int find_right(int x){
    int l = 0,r = n - 1;
    while(l < r){
        int mid = l + r + 1 >> 1;
        if(q[mid] <= x) l = mid;
        else r = mid - 1;
    }
    return l;
}

int main(){
    cin >> n;
    for(int i = 0;i < n;i++) cin >> q[i];
    sort(q,q + n);
    int mid = q[(n - 1) / 2];
    int l = find_left(mid),r = find_right(mid);
    if(l == n - 1 - r) cout << mid << endl;
    else cout << -1 << endl;
    return 0;
}