算法总结——主席树（poj2104）

#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<algorithm>
#include<cstring>
#include<string>
#include<cmath>
#include<ctime>
#include<cctype>
using namespace std;
const int N=100010;
int rank[N],root[N];
int n,m,tot;
struct node
{
  int x,id;
}a[N];
inline bool operator<(node a,node b)
{
  return a.x<b.x;
}
struct tree
{
  int l;
  int r;
  int sum;
}tr[N*20];
void build(int num,int &x,int l,int r)
{
  tr[tot++]=tr[x],x=tot-1;
  tr[x].sum++;
  if(l==r)  return;
  int mid=(l+r)/2;
  if(num<=mid)  build(num,tr[x].l,l,mid);
  else build(num,tr[x].r,mid+1,r);
}
int query(int i,int j,int k,int l,int r)
{
  if(l==r)  return l;
  int mid=(l+r)/2;
  int T=tr[tr[j].l].sum-tr[tr[i].l].sum;
  if(k<=T) return query(tr[i].l,tr[j].l,k,l,mid);
  else return query(tr[i].r,tr[j].r,k-T,mid+1,r);
}
int main()
{
  //freopen("a.in","r",stdin);
  int d,b,c;
  while(scanf("%d%d",&n,&m)!=EOF)
  {
    for(int i=1;i<=n;i++)
    {  
      scanf("%d",&a[i].x);
      a[i].id=i;
    }
    sort(a+1,a+n+1);
    for(int i=1;i<=n;i++)
      rank[a[i].id]=i;
    tot=1;
    for(int i=1;i<=n;i++)
    {
      root[i]=root[i-1];
      build(rank[i],root[i],1,n);
    }
    while(m--)
    {
      scanf("%d%d%d",&d,&b,&c);  
      cout<<a[query(root[d-1],root[b],c,1,n)].x<<endl;
    } 
  }
  return 0;
}