poj-1442 Black Box(Treap)

题目链接：

Black Box

题意：

给一个序列,m个询问,每个询问是求前x个数中的第i小是多少;

思路：

Treap的入门题目;Treap能实现STL的set实现不了的功能,如名次树(rank tree)rank tree 支持两种新操作,一个是找出第k小元素,一个是找出x的的rank;

AC代码：

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <cmath>
//#include <bits/stdc++.h>
#include <stack>
#include <map>
 
using namespace std;
 
#define For(i,j,n) for(int i=j;i<=n;i++)
#define mst(ss,b) memset(ss,b,sizeof(ss));
 
typedef  long long LL;
 
template<class T> void read(T&num) {
    char CH; bool F=false;
    for(CH=getchar();CH<'0'||CH>'9';F= CH=='-',CH=getchar());
    for(num=0;CH>='0'&&CH<='9';num=num*10+CH-'0',CH=getchar());
    F && (num=-num);
}
int stk[70], tp;
template<class T> inline void print(T p) {
    if(!p) { puts("0"); return; }
    while(p) stk[++ tp] = p%10, p/=10;
    while(tp) putchar(stk[tp--] + '0');
    putchar('\n');
}
 
const LL mod=1e9+7;
const double PI=acos(-1.0);
const int inf=1e9+10;
const int N=1e6+10;
const int maxn=1e3+20;
const double eps=1e-12;

int a[N];
struct Treap
{
    Treap *ch[2];
    int siz,key,fix;
    Treap(int key)
    {
        siz=1;
        fix=rand();
        this->key=key;
        ch[0]=ch[1]=NULL;
    }
    int cmp(int x) const
    {
        if(x==key)return -1;
        return x<key? 0:1;
    }
    void maintain()
    {
        siz=1;
        if(ch[0]!=NULL)siz+=ch[0]->siz;
        if(ch[1]!=NULL)siz+=ch[1]->siz;
    }
};

void Rotate(Treap* &o,int d)
{
    Treap* k=o->ch[d^1];
    o->ch[d^1]=k->ch[d];
    k->ch[d]=o;
    o->maintain();
    k->maintain();
    o=k;
}

void insert(Treap* &o,int x)
{
    if(o==NULL)o=new Treap(x);
    else 
    {
        int d=(x<o->key? 0:1);
        insert(o->ch[d],x);
        if(o->ch[d]->fix > o->fix)Rotate(o,d^1);
    }
    o->maintain();
}
int  getans(Treap* &o,int x)
{
    //cout<<o->key<<"*****\n";
    if(o==NULL||x<=0||x> o->siz)return 0;
    int s=(o->ch[0] == NULL? 0: o->ch[0]->siz);
    if(x==s+1)return o->key;
    else if(x<=s)return getans(o->ch[0],x);
    else return getans(o->ch[1],x-s-1);
}
void removetree(Treap* &o)
{
    if(o->ch[0]!=NULL)removetree(o->ch[0]);
    if(o->ch[1]!=NULL)removetree(o->ch[1]);
    delete o;
    o=NULL;
}

int main()
{
    int n,m;
    while(scanf("%d%d",&n,&m)!=EOF)
    {
        For(i,1,n)read(a[i]);
        int pre=1,x;
        Treap* root=NULL;
        For(i,1,m)
        {
            read(x);
            for(int j=pre;j<=x;j++)insert(root,a[j]);
            printf("%d\n",getans(root,i));
            pre=x+1;
        }
        removetree(root);
    }
    return 0;
}