Chairman tree review

Chairman tree : Persistent weight segment tree
Templates （ Query interval No k Big , Single point modification ）：luogu【 Templates 】 Persistent line tree 2
Using the idea of prefix and , Dynamic opening point
Use the existing and unchanging parts to reduce the space complexity

#include<bits/stdc++.h>
#define ll long long
#define maxn 200010
using namespace std;
int n,m;
ll a[maxn];// Original array  
ll b[maxn];// Discrete array 
int tot;
struct node{
    
	int l,r,sum;
}e[maxn*50]; 
int cnt,rot[maxn];// Root node array  
void insert(int l,int r,int pre,int &now,int k){
    
	e[++cnt]=e[pre];
	now=cnt;
	e[now].sum++;
	if(l==r)return ;
	int mid=(l+r)>>1;
	if(k<=mid)insert(l,mid,e[pre].l,e[now].l,k);
	else insert(mid+1,r,e[pre].r,e[now].r,k); 
}
int query(int l,int r,int L,int R,int k){
    
	if(l==r){
    
		return l;
	}
	int mid=(l+r)>>1;
	int res=e[e[R].l].sum-e[e[L].l].sum;
	if(k<=res)return query(l,mid,e[L].l,e[R].l,k);
	else return query(mid+1,r,e[L].r,e[R].r,k-res);
}
int main(){
    
	scanf("%d%d",&n,&m);
	for(int i=1;i<=n;i++){
    
		scanf("%lld",&a[i]);
		b[i]=a[i];
	}
	sort(b+1,b+n+1);
	tot=unique(b+1,b+n+1)-b-1;
	for(int i=1;i<=n;i++){
    
		int k=lower_bound(b+1,b+tot+1,a[i])-b;
		insert(1,n,rot[i-1],rot[i],k);
	}
	for(int L,R,k,i=1;i<=m;i++){
    
		scanf("%d%d%d",&L,&R,&k);
		cout<<b[query(1,n,rot[L-1],rot[R],k)]<<endl;
	}
	return 0;
}