1057 Stack (30 分)

1057 Stack (30 分)

#include <iostream>
#include <stack>
using namespace std;
const int N = 1e5 + 10;
int a[N];
stack<int> s;
int lowbit(int x){
	return x & -x;
}
void add(int x, int y){
	for(int i = x; i < N; i += lowbit(i))
		a[i] += y;
}
int sum(int x){
	int ans = 0;
	for(int i = x; i >= 1; i -= lowbit(i))
		ans += a[i];
	return ans;
}
int main(){
	int n, x;
	cin >> n;
	while(n--){
		string s1;
		cin >> s1; 
		if(s1 == "Push"){
			cin >> x;
			add(x, 1);
			s.push(x);		
		}
		else if(s1 == "Pop"){
			if(s.empty())
				cout << "Invalid" << endl;
			else{
				cout << s.top() << endl;	
				add(s.top(), -1);
				s.pop();
			}
		}
		else{
			if(s.empty())
				cout << "Invalid" << endl;
			else{
				int l = 1, r = N, m = (s.size() + 1) / 2, ans;
				while(l <= r){
					int mid = (l + r) / 2;
					if(sum(mid) < m)
						l = mid + 1;
					else{
						ans = mid;
						r = mid - 1;
					}
				}
				cout << ans << endl;
			}
		}
	}
}