思路： 树状数组，每插入一个就对相应位置更新。 代码：
#include <bits/stdc++.h> using namespace std; typedef long long ll; #define endl '\n' stack<int>s; const int maxn = 100010; int c[maxn]; int lowbit(int x) { return x & (-x); } void update(int x, int p) { for (int i = x; i < maxn; i += lowbit(i)) c[i] += p; } int getsum(int x) { int s = 0; for (int i = x; i >= 1; i -= lowbit(i)) s += c[i]; return s; } void peek() { int l = 1, r = maxn, mid, k = (s.size() + 1) / 2; while (l < r) { mid = (l + r) / 2; if (getsum(mid) >= k) { r = mid; } else { l = mid + 1; } } cout << l << endl; } int main() { int n, temp; cin >> n; string ord; while (n--) { cin >> ord; if (ord[1] == 'u') { cin >> temp; s.push(temp); update(temp, 1); } else if (ord[1] == 'o') { if (!s.empty()) { update(s.top(), -1); cout << s.top() << endl; s.pop(); } else cout << "Invalid" << endl; } else if (ord[1] == 'e') { if (!s.empty()) peek(); else cout << "Invalid" << endl; } } return 0; }