栈

https://www.acwing.com/problem/content/description/1545/

#include <iostream>
#include <cstring>
#include <set>
#include <stack>

using namespace std;

stack<int> stk;
multiset<int> up, down;

void adjust()
{
    while (up.size() > down.size())
    {
        down.insert(*up.begin());
        up.erase(up.begin());
    }

    while (down.size() > up.size() + 1)
    {
        auto it = down.end();
        it -- ;
        up.insert(*it);
        down.erase(it);
    }
}

int main()
{
    int n;
    scanf("%d", &n);
    char op[20];
    while (n -- )
    {
        scanf("%s", op);
        if (strcmp(op, "Push") == 0)
        {
            int x;
            scanf("%d", &x);
            stk.push(x);
            if (up.empty() || x < *up.begin()) down.insert(x);
            else up.insert(x);
            adjust();
        }
        else if (strcmp(op, "Pop") == 0)
        {
            if (stk.empty()) puts("Invalid");
            else
            {
                int x = stk.top();
                stk.pop();
                printf("%d\n", x);
                auto it = down.end();
                it -- ;
                if (x <= *it) down.erase(down.find(x));
                else up.erase(up.find(x));

                adjust();
            }
        }
        else
        {
            if (stk.empty()) puts("Invalid");
            else
            {
                auto it = down.end();
                it -- ;
                printf("%d\n", *it);
            }
        }
    }

    return 0;
}

树状数组的做法

#include <iostream>
#include <cstring>
#include <algorithm>
#include <stack>

using namespace std;

const int N = 100010;

int tr[N];
stack<int> s;

int lowbit(int x)
{
    return x & -x;
}

void add(int x, int c)
{
    for(int i = x; i <= N; i += lowbit(i)) tr[i] += c;
}

int getsum(int x)
{
    int sum = 0;
    for(int i = x; i; i -= lowbit(i)) sum += tr[i];
    return sum;
}

void PeekMedian() 
{
    int left = 1, right = N, k = (s.size() + 1) >> 1;
    while(left < right) 
    {
        int mid = (left + right) >> 1;
        if(getsum(mid) >= k) right = mid;
        else left = mid + 1;
    }
    printf("%d\n", left);
}

int main()
{
    int n, temp;
    scanf("%d", &n);
    char str[15];
    for(int i = 0; i < n; i ++ )
    {
        scanf("%s", str);
        if(str[1] == 'u') 
        {
            scanf("%d", &temp);
            s.push(temp);
            add(temp, 1);
        } 
        else if(str[1] == 'o') 
        {
            if(!s.empty())
            {
                add(s.top(), -1);
                printf("%d\n", s.top());
                s.pop();
            }
            else printf("Invalid\n");
        }
        else if(!s.empty()) PeekMedian();
        else printf("Invalid\n");
    }
    return 0;
}