HDU

题意：

求区间第K小

思路：

主席树

（此题数据较水，建议同时通过 POJ 2104 HDU 2665）

代码：

#include <iostream>#include <cstdio>#include <vector>#include <algorithm>#include <cstring>using namespace std;#define ls l,mid#define rs mid+1,r#define mi (l+r)/2const int MAXN=1e5+7;vector <int> a;typedef struct Node{    int val;    int son[2];}Node;Node tree[MAXN*20];int b[MAXN];int cnt,pos,rtid[MAXN],ans;int new_node(){    int rt=++cnt;    tree[rt].val=0;    tree[rt].son[0]=tree[rt].son[1]=0;    return rt;}int build(int l,int r){    int rt=new_node();    if(l==r){        return rt;    }    int mid=mi;    tree[rt].son[0]=build(ls);    tree[rt].son[1]=build(rs);    return rt;}void update(int l,int r,int rt,int lart){    tree[rt].val=tree[lart].val;    tree[rt].val++;    if(l==r) return ;    int mid=mi;    if(pos>mid){        tree[rt].son[0]=tree[lart].son[0];        tree[rt].son[1]=new_node();        update(mid+1,r,tree[rt].son[1],tree[lart].son[1]);    }else{        tree[rt].son[1]=tree[lart].son[1];        tree[rt].son[0]=new_node();        update(l,mid,tree[rt].son[0],tree[lart].son[0]);    }}int query(int l,int r,int rt,int lart,int k){    if(l==r) return l;    int mid=mi;    int sum=tree[tree[rt].son[0]].val-tree[tree[lart].son[0]].val;    if(sum>=k) return query(ls,tree[rt].son[0],tree[lart].son[0],k);    else return query(rs,tree[rt].son[1],tree[lart].son[1],k-sum);}void ini(){    a.clear();cnt=0;}int main(){    int n,m,st,en,k,T;    scanf("%d",&T);    while(T--){        scanf("%d%d",&n,&m);        ini();        for(int i=1;i<=n;i++) scanf("%d",&b[i]),a.push_back(b[i]);        sort(a.begin(),a.end());a.erase(unique(a.begin(),a.end()),a.end());        int len=a.size();        rtid[0]=build(1,len);        for(int i=1;i<=n;i++){            pos=lower_bound(a.begin(),a.end(),b[i])-a.begin()+1;            rtid[i]=++cnt;            update(1,len,rtid[i],rtid[i-1]);        }        for(int i=1;i<=m;i++){            scanf("%d%d%d",&st,&en,&k);            ans=query(1,len,rtid[en],rtid[st-1],k);            printf("%d\n",a[ans-1]);        }    }}