COGS 930. [河南省队2012] 找第k小的数 主席树

#include<cstdio>
#include<iostream>
#include<algorithm>
#define MAXN 100005
#define MAX 2000005
using namespace std;
int sum[MAX],l[MAX],mid[MAX],r[MAX],a[MAXN],b[MAXN],n,m,sz,size,root[MAXN],cnt;
void build(int &x,int z,int y)
{
     x=++sz;
     if(z==y){mid[sz]=-y;return;}
     mid[sz]=(z+y)>>1;
     build(l[x],z,mid[x]);
     build(r[x],mid[x]+1,y);
}
void update(int &x,int last,int to)
{
     x=++sz;
     mid[sz]=mid[last];
     sum[sz]=sum[last]+1;
     if(mid[sz]<0)return;
     l[sz]=l[last];
     r[sz]=r[last];
     if(to<=mid[x])update(l[x],l[last],to);
     else update(r[x],r[last],to);
}
int query(int x,int y,int k)
{
     if(mid[y]<0)return b[-mid[y]];
     cnt=sum[l[y]]-sum[l[x]];
     if(k<=cnt)return query(l[x],l[y],k);
     else return query(r[x],r[y],k-cnt);
}
inline void work()
{
     int x,y,k;
     scanf("%d%d%d",&x,&y,&k);
     printf("%d\n",query(root[x-1],root[y],k));
}
int main()
{
    freopen("kth.in","r",stdin);
    freopen("kth.out","w",stdout);
      scanf("%d%d",&n,&m);
      for(int i=1;i<=n;i++){scanf("%d",&a[i]);b[i]=a[i];}
      sort(b+1,b+n+1);
      size=unique(b+1,b+n+1)-(b+1);
      build(root[0],1,size);
      for(int i=1;i<=n;i++){a[i]=lower_bound(b+1,b+size+1,a[i])-b;update(root[i],root[i-1],a[i]);}
      while(m--)work();
    return 0;
}