P2201 数列编辑器

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#include<iostream>
#include<algorithm>
using namespace std;
#define re register
const int MAXn = 1e6;

template <class T>
inline void read(T &a) {
    register char c;while (c = getchar(), (c < '0' || c > '9') && c != '-');register bool f = c == '-';register T x = f ? 0 : c - '0';while (c = getchar(), c >= '0' && c <= '9') {x = (x << 3) + (x << 1) + (c ^ 48);}a = f ? -x : x;
}

struct Ele {
    int val, sum, maxsum;
    Ele(){}
    Ele(int val_):val(val_){}
};
struct Stack {
    int top;
    Ele stk[MAXn + 10];
    inline void Push(int x) {
        stk[++top] = Ele(x);
    }
    inline int Pop() {
        return stk[top--].val;
    }
}fro, beh;
int n;

int main() {
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    cin >> n;
    char opt;
    for (re int i = 1, x; i <= n; ++i) {
        cin >> opt;
        switch (opt) {
        case 'I':
            cin >> x;
            fro.Push(x);
            fro.stk[fro.top].sum = fro.stk[fro.top - 1].sum + fro.stk[fro.top].val;
            fro.stk[fro.top].maxsum = (fro.top == 1) ? fro.stk[fro.top].sum :
                                                max(fro.stk[fro.top - 1].maxsum, fro.stk[fro.top].sum);
            break;
        case 'D':
            fro.Pop();
            break;
        case 'L':
            beh.Push(fro.Pop());
            break;
        case 'R':
            fro.Push(beh.Pop());
            fro.stk[fro.top].sum = fro.stk[fro.top - 1].sum + fro.stk[fro.top].val;
            fro.stk[fro.top].maxsum = (fro.top == 1) ? fro.stk[fro.top].sum :
                                                max(fro.stk[fro.top - 1].maxsum, fro.stk[fro.top].sum);
            break;
        case 'Q':
            cin >> x;
            cout << fro.stk[x].maxsum << '\n';
        }
    }
}