rt,这里用的是oiwiki里的合并实现平衡维护，在倒数第二个点mle了。

#include<iostream>
#include<cstdio>
#include<cstring>
#define lc(u) (tr[u].lc)
#define rc(u) (tr[u].rc)
#define W(u) (tr[u].leaf)
#define val(u) (tr[u].val)
#define del(u) (pts[++cur]=(u))
#define pd(w1,w2) (min(w1,w2)>=alp*((w1)+(w2)))
//#define pd(w1,w2) ((w1)<=3*(w2)||(w2)<=3*(w1))
using namespace std;

const int N=1e5+10;
const double alp=0.292;
struct Node{int lc,rc,leaf,val;}tr[(N<<1)+20];
static int pts[N];
int tot,cur,n,m,ans,rt;

inline int max(int x,int y){return x>y?x:y;}
inline int min(int x,int y){return x<y?x:y;}
void fastio(){
	ios::sync_with_stdio(0);
	cin.tie(0);cout.tie(0);
}

inline int New(int v){
	int u=cur?pts[cur--]:(++tot);
	tr[u]={0,0,1,v};
	return u;
}
inline void pushup(int u){
	W(u)=W(lc(u))+W(rc(u));
	val(u)=max(val(lc(u)),val(rc(u)));
}
int merge(int x,int y){
	if(!x||!y) return x^y;
	if(pd(W(x),W(y))){
		int u=New(0);
		lc(u)=x;rc(u)=y;
		return pushup(u),u;
	}
	if(W(x)>=W(y)){
		if(pd(W(lc(x)),W(rc(x))+W(y)))
			return rc(x)=merge(rc(x),y),pushup(x),x;
		lc(x)=merge(lc(x),lc(rc(x)));
		del(rc(x));rc(x)=merge(rc(rc(x)),y);
		return pushup(x),x;
	}
	if(pd(W(x)+W(lc(y)),W(rc(y))))
		return lc(y)=merge(x,lc(y)),pushup(y),y;
	rc(y)=merge(rc(lc(y)),rc(y));
	del(lc(y));lc(y)=merge(x,lc(lc(y)));
	return pushup(y),y;
}
inline void maintain(int &u){
	if(W(u)==1)return;
	if(pd(W(lc(u)),W(rc(u))))return;
	int x=lc(u),y=rc(u);
	del(u);u=merge(x,y);
}

void insert(int &u,int v){
	if(!rt){rt=New(v);return;}
	if(W(u)==1){
		lc(u)=New(val(u)),rc(u)=New(v);
		if(val(u)>v)lc(u)^=rc(u)^=lc(u)^=rc(u);
		return pushup(u);
	}
	if(val(lc(u))>=v)insert(lc(u),v);
	else insert(rc(u),v);
	pushup(u);maintain(u);
}
void erase(int &u,int v) {
	if(!u)return;
	if (val(lc(u))>=v){
		if(W(lc(u))==1)del(u),u=rc(u);
		else erase(lc(u),v),pushup(u),maintain(u);
	}
	else {
		if(W(rc(u))==1)del(u),u=lc(u);
		else erase(rc(u),v),pushup(u),maintain(u);
	}
}
inline int Rank(int v){
	int u=rt,ans=0;
	while(W(u)!=1){
		if(val(lc(u))>=v)u=lc(u);
		else ans+=W(lc(u)),u=rc(u);
	}
	return ans+1;
}
inline int kth(int k){
	int u=rt;
	while(W(u)!=1){
		if(W(lc(u))>=k)u=lc(u);
		else k-=W(lc(u)),u=rc(u);
	}
	return val(u);
}
inline int pre(int v){
	int u=rt;
	while(W(u)!=1){
		if(val(lc(u))>=v)u=lc(u);
		else ans=val(lc(u)),u=rc(u);
	}
	if(val(u)>=v)return ans;
	else return val(u);
}
inline int nxt(int v){
	int u=rt;
	while(W(u)!=1){
		if(val(lc(u))<=v)u=rc(u);
		else ans=val(rc(u)),u=lc(u);
	}
	if(val(u)<=v)return ans;
	else return val(u);
}

signed main(){
	fastio();
	cin>>m;
	int ch,x;
	for(int i=1;i<=m;i++){
		cin>>ch>>x;
		if(ch==1)insert(rt,x);
		if(ch==2)erase(rt,x);
		if(ch==3)cout<<Rank(x)<<"\n";
		if(ch==4)cout<<kth(x)<<"\n";
		if(ch==5)cout<<pre(x)<<"\n";
		if(ch==6)cout<<nxt(x)<<"\n";
	}
	return 0;
}

感谢各位大佬的帮助。