PAT (Advanced) 1057. Stack (30)

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

using namespace std;

multiset<int> small, big;
stack<int> s;
int mid;

void adjust()
{
	if (small.size() > big.size() + 1)
	{
		auto it = small.end();
		--it;
		big.insert(*it);
		small.erase(it);
	}
	else if (small.size() < big.size())
	{
		auto it = big.begin();
		small.insert(*it);
		big.erase(it);
	}
	if (s.size() > 0)
	{
		auto it = small.end();
		--it;
		mid = *it;
	}
}

int main()
{
	int n;
	char op[15];
	int top, Key;
	scanf("%d", &n);
	while (n--)
	{
		scanf("%s", op);
		if (op[1] == 'o')
		{
			if (s.size() == 0)
				printf("Invalid\n");
			else
			{
				top = s.top();
				s.pop();
				printf("%d\n", top);
				if (mid >= top)
				{
					auto it = small.find(top);
					small.erase(it);
				}
				else
				{
					auto it = big.find(top);
					big.erase(it);
				}
				adjust();
			}
		}
		else if (op[1] == 'u')
		{
			scanf("%d", &Key);
			if (s.size() == 0)
			{
				small.insert(Key);
				mid = Key;
			}
			else if (Key <= mid)
				small.insert(Key);
			else
				big.insert(Key);
			s.push(Key);
			adjust();
		}
		else if (op[1] == 'e')
		{
			if (s.size() == 0)
				printf("Invalid\n");
			else
				printf("%d\n", mid);
		}
	}
}