可持久化线段树（主席树）

/*    可持久优化线段树    POJ2104 区间第k大值 k-th number    by sbn    2017-12-21 */#include<iostream>#include<cstdio>#include<vector>#include<cstdlib>#include<algorithm>using namespace std;typedef long long ll;struct node{    int l,r;    int num;    //存放第i个值在原序列排第几     int sum;    //线段树结点里保存的值 }   tree[5000001];int n,m,tot=0,head[5000001];//每个结点在线段树中标号 int san[5000001]; //存放原有值的排序 void build(int l,int r,int &u){    u=++tot;    tree[u].sum=0;    if (l==r)   return;    build(l,(l+r)>>1,tree[u].l);    build((l+r)/2+1,r,tree[u].r);}//一棵空树 void update(int last,int u,int l,int r,int &x){ //     x=++tot;    tree[x].l=tree[last].l;    tree[x].r=tree[last].r;    tree[x].sum=tree[last].sum+1;    if (l==r)   return;    int mid=(l+r)/2;    if(u<=mid)  update(tree[last].l,u,l,mid,tree[x].l);    else update(tree[last].r,u,mid+1,r,tree[x].r);}int query(int s,int t,int l,int r,int k){    if (l==r)   return l;    int mid=(l+r)/2;    int cnt=tree[tree[t].l].sum-tree[tree[s].l].sum;    if (k<=cnt)        return query(tree[s].l,tree[t].l,l,mid,k);        return query(tree[s].r,tree[t].r,mid+1,r,k-cnt);}inline void debug(){        cout<<"san:"<<endl;    for (int i=1;i<=n;i++)  cout<<san[i]<<" ";cout<<endl;    cout<<"num:"<<endl;    for (int i=1;i<=n;i++)  cout<<tree[i].num<<" ";cout<<endl;    cout<<"sum:"<<endl;    for (int i=1;i<=n;i++)  cout<<tree[i].sum<<" ";cout<<endl;    cout<<"head:"<<endl;    for (int i=0;i<=n;i++)  cout<<head[i]<<" ";cout<<endl;    cout<<"l,r"<<endl;    for (int i=0;i<=n;i++)  cout<<tree[head[i]].l<<","<<tree[head[i]].r<<" ";cout<<endl;}int main(){    int x,y,z;    cin>>n>>m;    for (int i=1;i<=n;i++)    {        cin>>san[i];        tree[i].num=san[i];    }    sort(san+1,san+n+1);    int cnt=unique(san+1,san+n+1)-san-1;    //去重     cout<<cnt<<endl;    build(1,cnt,head[0]);    debug();    update(head[0],tree[1].num,1,cnt,head[1]);    debug();    for (int i=1;i<=n;i++)        tree[i].num=lower_bound(san+1,san+n+1,tree[i].num)-san;    for (int i=1;i<=n;i++)        update(head[i-1],tree[i].num,1,cnt,head[i]);    //debug();    for (int i=1;i<=m;i++){        cin>>x>>y>>z;        cout<<san[query(head[x-1],head[y],1,cnt,z)]<<endl;    }    return 0;}