主席树 初见

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
struct Node{
    Node *ch[2];
    int a,siz;
    Node (){ch[0]=ch[1]=NULL;siz=a=0;}
    void updata(){
        siz=a;
        if (ch[0]!=NULL) siz+=ch[0]->siz;
        if (ch[1]!=NULL) siz+=ch[1]->siz;
    }
}*null=new Node(),*root[100001]={NULL},q[2000001];
int a[100001]={0},a2[100001]={0},q_size=0;
void insert(Node *&y,Node *&x,int l,int r,int xx)
{
    int mid;
    if (x==NULL) x=null;
    y=&q[++q_size];
    *y=Node();
    if (l==r)
    {
        *y=*x;
        y->siz++;y->a++;
        return;
    }
    mid=(l+r)/2;
    if (xx<=a2[mid])
    {
        insert(y->ch[0],x->ch[0],l,mid,xx);
        y->ch[1]=x->ch[1];
        y->updata();
    }
    else
    {
        insert(y->ch[1],x->ch[1],mid+1,r,xx);
        y->ch[0]=x->ch[0];
        y->updata();
    }
}
void find(Node *&x2,Node *&x1,int l,int r,int k)
{
    int ss=0,mid;
    if (x2==NULL) x2=null;
    if (x1==NULL) x1=null;
    if (l==r)
    {
        printf("%d\n",a2[l]);
        return;
    }
    mid=(l+r)/2;
    if (x2->ch[0]!=NULL) ss+=x2->ch[0]->siz;
    if (x1->ch[0]!=NULL) ss-=x1->ch[0]->siz;
    if (ss>=k) find(x2->ch[0],x1->ch[0],l,mid,k);
    else find(x2->ch[1],x1->ch[1],mid+1,r,k-ss);
}
int main()
{
    freopen("kth.in","r",stdin);
    freopen("kth.out","w",stdout);
    
    int n,m,i,j,k,l,r;
    null->ch[0]=null;null->ch[1]=null;
    scanf("%d%d",&n,&m);
    for (i=1;i<=n;++i)
    {
        scanf("%d",&a[i]);
        a2[i]=a[i];
    }
    sort(a2+1,a2+n+1);
    int size;
    size=unique(a2+1,a2+n+1)-a2-1;
    for (i=1;i<=n;++i)
        insert(root[i],root[i-1],1,size,a[i]);
    for (i=1;i<=m;++i)
    {
        scanf("%d%d%d",&l,&r,&k);
        find(root[r],root[l-1],1,size,k);
    }
    
    fclose(stdin);
    fclose(stdout);
}