题目链接

  思路：

  用stack模拟输入输出，用vector每次按顺序插入值，用lower_bound()函数

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int maxn = 1e4+10;
const int INF=0x3f3f3f3f;
#define fir first
#define sec second

int main()
{
    int n;
    cin>>n;
    stack<int>st;
    vector<int>vc;
    vector<int>::iterator it;
    while(n--){
        string s;
        cin>>s;
        int x;
        if(s[1]=='o'){
            if(vc.size()==0) cout<<"Invalid"<<endl;
            else{
                it=lower_bound(vc.begin(),vc.end(),st.top());
                cout<<(*it)<<endl;
                vc.erase(it);
                st.pop();
            }
        }
        else if(s[1]=='u'){
            cin>>x;
            it=lower_bound(vc.begin(),vc.end(),x);
            vc.insert(it,x);
            st.push(x);
        }
        else{
            int k=vc.size();
            if(!k) cout<<"Invalid"<<endl;
            else if(k%2) cout<<vc[k/2]<<endl;
            else cout<<vc[(k/2)-1]<<endl;
        }
    }
    return 0;
}