编辑器题目【栈。对顶堆】

用两个栈来模拟光标的移动，sum来维护A栈的前缀和，f用来维护最大值。

#include <iostream>
#include <algorithm>
#include <stack>
using namespace std;
stack<int> A, B;
constexpr size_t N = 1e6 + 5;
int sum[N], f[N];

int main() {
    int m;
    cin >> m;
    f[0] = -1e7;//初始化一定记住
    while (m --) {
        char cmd;
        int x;
        cin >> cmd;
        if (cmd == 'I') {
            cin >> x;
            A.push(x);
            sum[A.size()] = sum[A.size() - 1] + x;
            f[A.size()] = max(f[A.size() - 1], sum[A.size()]);
        }
        if (cmd == 'D')
            if (!A.empty())
                A.pop();

        if (cmd == 'L') {
            if(!A.empty()) {
                B.push(A.top());
                A.pop();
            }
        }

        if (cmd == 'R') {
            if (!B.empty()) {
                A.push(B.top());
                B.pop();
                sum[A.size()] = sum[A.size() - 1] + A.top();
                f[A.size()] = max(f[A.size() - 1], sum[A.size()]);
            }
        }

        if (cmd == 'Q') {
            cin >> x;
            cout << f[x] << endl;
        }
    }
    return 0;
}