1057 Stack

//
//map
//20    1
//25    2
//30    2   mid
//35    2
//beforemid 3
//stack
//beforemid必须小于（长度+1）/2

#include<iostream>
#include<map>
#include<stack>
using namespace std;

map<int,int>M;
map<int,int>::iterator mid;
int beforemid;
stack<int>S;

void adjust_mid(){
    if(beforemid>=(S.size()+1)/2){//左移
        mid--;
        beforemid-=mid->second;
    }
    else if(beforemid+mid->second<(S.size()+1)/2){//右移
        beforemid+=mid->second;
        mid++;
    }
}

int main()
{
    int N;
    cin>>N;
    while(N--){
        string command;
        cin>>command;
        
        if(command[1]=='u'){//push操作
            int key;
            cin>>key;
            S.push(key);
            M[key]++;
            if(S.size()==1){//初始化
                mid=M.begin();
                beforemid=0;
            }
            else{
                 if(key<mid->first)beforemid++;
                adjust_mid();
            }
        }
        else if(command[1]=='o'){//pop操作
            if(!S.size()){
                cout<<"Invalid\n";
            }
            else{
                int key;
                key=S.top();
                cout<<key<<endl;
                S.pop();
                M[key]--;
                if(key<mid->first)beforemid--;
                adjust_mid();
                if(M[key]==0){
                    if(mid->first==key)mid++;
                    M.erase(key);
                }
            }
        }
        else {//peekmedian
            if(!S.size()){
                cout<<"Invalid\n";
            }
            else{
                cout<<mid->first<<endl;
            }
        }
    }
}