COGS 930. [河南省队2012] 找第k小的数

#include <algorithm>
#include <cstdio>
#define N 100500
using namespace std;
struct cmt
{
    int l,r,size;
}tr[N*30];
int n,m,rt[N],tot,a[N],b[N],size;
int build(int l,int r)
{
    int now=++tot;
    tr[now].size=0;
    if(l==r) return now;
    int mid=(l+r)>>1;
    tr[now].l=build(l,mid);
    tr[now].r=build(mid+1,r);
    return now;
}
inline int Rank(int x)
{
    return lower_bound(b+1,b+1+size,x)-b;
}
void update(int l,int r,int x,int &y,int t)
{
    y=++tot;
    tr[y].size=tr[x].size+1;
    if(l==r) return;
    tr[y].l=tr[x].l;
    tr[y].r=tr[x].r;
    int mid=(l+r)>>1;
    if(t<=mid) update(l,mid,tr[x].l,tr[y].l,t);
    else update(mid+1,r,tr[x].r,tr[y].r,t);
}
int ask(int l,int r,int lx,int rx,int k)
{
    if(l==r) return l;
    int mid=(l+r)>>1;
    if(tr[tr[rx].l].size-tr[tr[lx].l].size>=k) return ask(l,mid,tr[lx].l,tr[rx].l,k);
    else {k-=tr[tr[rx].l].size-tr[tr[lx].l].size;return ask(mid+1,r,tr[lx].r,tr[rx].r,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+1+n);
    size=unique(b+1,b+1+n)-b-1;
    rt[0]=build(1,size);
    for(int i=1;i<=n;++i) update(1,size,rt[i-1],rt[i],Rank(a[i]));
    for(int l,r,k;m--;)
    {
        scanf("%d%d%d",&l,&r,&k);
        printf("%d\n",b[ask(1,size,rt[l-1],rt[r],k)]);
    }
    return 0;
}
int sb=Main();
int main(int argc,char *argv[]) {;}