C++ :

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

using namespace std;

#define INF 0x3f3f3f3f
const int maxn = 1e6 + 7;
stack<int>s1,s2;
int f[maxn],sum[maxn];
int main()
{
    int Q;scanf("%d",&Q);
    sum[0] = 0;
    int cnt = 0;
    memset(f,-INF,sizeof(f));
    while(Q--)
    {
        char op[10];int n;
        scanf("%s",op);
        if(op[0] == 'I')
        {
            cnt++;
            scanf("%d",&n);
            sum[cnt] = sum[cnt - 1] + n;
            f[cnt] = max(f[cnt - 1],sum[cnt]);
            s1.push(n);
        }
        else if(op[0] == 'D')
        {
            if(!s1.empty())
            {
                cnt--;
                s1.pop();
            }
        }
        else if(op[0] == 'L')
        {
            if(!s1.empty())
            {
                cnt--;
                int tmp = s1.top();
                s2.push(tmp);
                s1.pop();
            }
        }
        else if(op[0] == 'R')
        {
            if(!s2.empty())
            {
                cnt++;
                int tmp = s2.top();
                sum[cnt] = sum[cnt - 1] + tmp;
                f[cnt] = max(f[cnt - 1],sum[cnt]);
                s1.push(tmp);
                s2.pop();
            }
        }
        else if(op[0] == 'Q')
        {
            scanf("%d",&n);
            printf("%d\n",f[n]);
        }
    }
    return 0;
}