注意点：
- 最小前缀和仅需要维护光标前的数值，因此sum[i](前缀和)及f[i](最小前缀和)可直接依托top_1(代表光标前数值的栈)存在.
#include<cstdio>
#include<iostream>
#include<cstring>
using namespace std;
const int MAXN=2e6,INF=2e9;
int stck_1[MAXN],top_1=0;//光标前
int f[MAXN];//最大前缀和
int id=0;
int sum[MAXN];//前缀和
int stck_2[MAXN],top_2=0;//光标后
int main(){
memset(f,0xcf,sizeof(f));
int q;
scanf("%d",&q);
for(int i=1;i<=q;i++){
char opt;
cin>>opt;
if(opt=='I'){//光标前插入数值
int x;
scanf("%d",&x);
stck_1[++top_1]=x;
sum[top_1]=sum[top_1-1]+x;
f[top_1]=max(f[top_1-1],sum[top_1]);
}else if(opt=='Q'){//查询最大前缀和
int x;
scanf("%d",&x);
printf("%d
",f[x]);
}else if(opt=='L'){//光标左移
if(!top_1)continue;
stck_2[++top_2]=stck_1[top_1--];
}else if(opt=='R'){//光标右移
if(!top_2)continue;
stck_1[++top_1]=stck_2[top_2--];
sum[top_1]=sum[top_1-1]+stck_1[top_1];
f[top_1]=max(f[top_1-1],sum[top_1]);
}else if(opt=='D'){//删除光标前一个元素
if(!top_1)continue;
top_1--;
}
}
return 0;
}