这是A之前的代码：

#include<bits/stdc++.h>
#define int long long
using namespace std;

class fhq{
	private: 
		struct node{
			int size,data,vis;
			node *l,*r;
		}*root=0,zero;
	public:
		node *newnode(int x){
			srand(time(0));
			node *ip=new node;
			ip->data=x,ip->vis=rand(),ip->size=1,ip->l=ip->r=NULL;
			return ip;
		}
		void pushup(node *ip){
			if(ip==NULL) return;
			ip->size=1;
			if(ip->l) ip->size+=ip->l->size;
			if(ip->r) ip->size+=ip->r->size;
		}
		void split(node *ip,int val,node *&l,node *&r){
			if(ip==NULL) l=r=NULL;
			else{
				if(ip->data<=val){
					l=ip;
					split(ip->r,val,ip->r,r);
				}
				else{
					r=ip;
					split(ip->l,val,l,ip->l);
				}
				pushup(ip);
			}
		}
		node* merge(node *&l,node *&r){
			if(l==NULL) return r;
			if(r==NULL) return l;
			if(l->vis<=r->vis){
				l->r=merge(l->r,r);
				pushup(l);
				return l;
			}
			r->l=merge(l,r->l);
			pushup(r);
			return r;
		}
		void add(int x){
			node *a=NULL,*b=NULL;
			split(root,x,a,b);
			node *tmp1=newnode(x),*tmp2=merge(a,tmp1);
			root=merge(tmp2,b);
		}
		void del(int x){
			node *a=NULL,*b=NULL,*c=NULL;
			split(root,x,b,c);
			split(b,x-1,a,b);
			node *tmp1=merge(b->l,b->r),*tmp2=merge(a,tmp1);
			root=merge(tmp2,c);
			if(b) delete b;
		}
		int findth(int x){
			node *a=NULL,*b=NULL;
			split(root,x-1,a,b);
			int ans=1;
			if(a) ans+=a->size;
			root=merge(a,b);
			return ans;
		}
		int findnum(int th){
		    node *ip=root;
		    while(ip!=NULL){
		    	int ls=0,rs=0;
		    	if(ip->l!=NULL) ls=ip->l->size;
		    	if(ip->r!=NULL) rs=ip->r->size;
		        if(ls+1==th) break;
		        else if(ls>=th) ip=ip->l;
		        else th-=ls+1,ip=ip->r;
		    }
		    return ip->data;
		}
		int lst(int x){
			node *a=NULL,*b=NULL,*ans=NULL;
			split(root,x-1,a,b);
			for(ans=a;ans->r;ans=ans->r);
			root=merge(a,b);
			return ans->data;
		}
		int nxt(int x){
			node *a=NULL,*b=NULL,*ans=NULL;
			split(root,x,a,b);
			for(ans=b;ans->l;ans=ans->l);
			root=merge(a,b);
			return ans->data;
		}
		void midfor(node *ip=0){
			if(!ip) ip=root;
			if(ip->l) midfor(ip->l);
			if(ip->r) midfor(ip->r);
		}
}t;
int n;

signed main(){
	cin>>n;
	while(n--){
		int opt,x;
		scanf("%lld%lld",&opt,&x);
		switch(opt){
			case 1:{t.add(x);break;}
			case 2:{t.del(x);break;}
			case 3:{printf("%lld\n",t.findth(x));break;}
			case 4:{printf("%lld\n",t.findnum(x));break;}
			case 5:{printf("%lld\n",t.lst(x));break;}
			case 6:{printf("%lld\n",t.nxt(x));break;}
		}
	}
	return 0;
}

这是A之后的代码（删的部分是注释掉的）：

#include<bits/stdc++.h>
#define int long long
using namespace std;

class fhq{
	private: 
		struct node{
			int size,data,vis;
			node *l,*r;
		}*root=0,zero;
	public:
		node *newnode(int x){
//			srand(time(0));，只有这么一处，而且这个地方应该是无关紧要的吧！
			node *ip=new node;
			ip->data=x,ip->vis=rand(),ip->size=1,ip->l=ip->r=NULL;
			return ip;
		}
		void pushup(node *ip){
			if(ip==NULL) return;
			ip->size=1;
			if(ip->l) ip->size+=ip->l->size;
			if(ip->r) ip->size+=ip->r->size;
		}
		void split(node *ip,int val,node *&l,node *&r){
			if(ip==NULL) l=r=NULL;
			else{
				if(ip->data<=val){
					l=ip;
					split(ip->r,val,ip->r,r);
				}
				else{
					r=ip;
					split(ip->l,val,l,ip->l);
				}
				pushup(ip);
			}
		}
		node* merge(node *&l,node *&r){
			if(l==NULL) return r;
			if(r==NULL) return l;
			if(l->vis<=r->vis){
				l->r=merge(l->r,r);
				pushup(l);
				return l;
			}
			r->l=merge(l,r->l);
			pushup(r);
			return r;
		}
		void add(int x){
			node *a=NULL,*b=NULL;
			split(root,x,a,b);
			node *tmp1=newnode(x),*tmp2=merge(a,tmp1);
			root=merge(tmp2,b);
		}
		void del(int x){
			node *a=NULL,*b=NULL,*c=NULL;
			split(root,x,b,c);
			split(b,x-1,a,b);
			node *tmp1=merge(b->l,b->r),*tmp2=merge(a,tmp1);
			root=merge(tmp2,c);
			if(b) delete b;
		}
		int findth(int x){
			node *a=NULL,*b=NULL;
			split(root,x-1,a,b);
			int ans=1;
			if(a) ans+=a->size;
			root=merge(a,b);
			return ans;
		}
		int findnum(int th){
		    node *ip=root;
		    while(ip!=NULL){
		    	int ls=0,rs=0;
		    	if(ip->l!=NULL) ls=ip->l->size;
		    	if(ip->r!=NULL) rs=ip->r->size;
		        if(ls+1==th) break;
		        else if(ls>=th) ip=ip->l;
		        else th-=ls+1,ip=ip->r;
		    }
		    return ip->data;
		}
		int lst(int x){
			node *a=NULL,*b=NULL,*ans=NULL;
			split(root,x-1,a,b);
			for(ans=a;ans->r;ans=ans->r);
			root=merge(a,b);
			return ans->data;
		}
		int nxt(int x){
			node *a=NULL,*b=NULL,*ans=NULL;
			split(root,x,a,b);
			for(ans=b;ans->l;ans=ans->l);
			root=merge(a,b);
			return ans->data;
		}
		void midfor(node *ip=0){
			if(!ip) ip=root;
			if(ip->l) midfor(ip->l);
			if(ip->r) midfor(ip->r);
		}
}t;
int n;

signed main(){
	cin>>n;
	while(n--){
		int opt,x;
		scanf("%lld%lld",&opt,&x);
		switch(opt){
			case 1:{t.add(x);break;}
			case 2:{t.del(x);break;}
			case 3:{printf("%lld\n",t.findth(x));break;}
			case 4:{printf("%lld\n",t.findnum(x));break;}
			case 5:{printf("%lld\n",t.lst(x));break;}
			case 6:{printf("%lld\n",t.nxt(x));break;}
		}
	}
	return 0;
}