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)/2 const 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]); } } }
