csp 201612-1 中间数

#include <bits/stdc++.h>
using namespace std;
const int N = 1001;
int main()
{
    int n;
    cin>>n;
    vector<int> a(n+1);
    for(int i=1;i<=n;++i)
        cin>>a[i];
    sort(a.begin()+1,a.end());
    if(n==1)
    {
        cout<<a[1]<<endl;
        return 0;
    }
    if(n%2)
    {
        int f=(n+1)/2;
        if(a[f]!=a[f-1]&&a[f]!=a[f+1])
            cout<<a[f]<<endl;
        else if(a[f]==a[f-1]&&a[f]==a[f+1])
        {
            int num = count(a.begin(),a.end(),a[f]);
            if(num%2==0)
                cout<<"-1"<<endl;
            else
            {
                num-=3;
                num/=2;
                if(f-1-num>0&&f+1+num<=n&&a[f-1-num]==a[f+1+num])
                    cout<<a[f]<<endl;
                else
                    cout<<"-1"<<endl;
            }
        }
        else
            cout<<"-1"<<endl;
    }
    else
    {
        if(a[n/2]==a[n/2+1])
        {
            int num = count(a.begin(),a.end(),a[n/2]);
            if(num%2)
                cout<<"-1"<<endl;
            else
            {
                num-=2;
                num/=2;
                if(n/2-num>0&&n/2+1+num<=n&&a[n/2-num]==a[n/2+1+num])
                    cout<<a[n/2]<<endl;
                else
                    cout<<"-1"<<endl;
            }
        }
        else
            cout<<"-1"<<endl;
    }
    return 0;
}