First
Second

快速排序还是很重要的

#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cstdio>
using namespace std;
int zn[5000001];
int n,Aimee;
void qs(int l,int r){
    int ll=l;int rr=r;int mid=zn[(l+r)/2];
    do{
        while(zn[rr]>mid){
            rr--;
        }
        while(zn[ll]<mid){
            ll++;
        }
        if(ll<=rr){
            swap(zn[ll],zn[rr]);
            ll++;
            rr--;
        }
    }while(ll<=rr);
    if(l<rr) qs(l,rr);
     if(r>ll)
    qs(ll,r);
}
int main(){
    scanf("%d",&n);
    for(int i=1;i<=n;++i){
        scanf("%d",&zn[i]);
    }
    qs(1,n); 
    for(int i=1;i<=n;++i){
        cout<<zn[i]<<" ";
    }
    return 0;
}

#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cstdio>
using namespace std;
int zn[5000001];
int n,Aimee;
void qs(int l,int r,int aim){
    int ll=l;int rr=r;int mid=zn[(l+r)/2];
    do{
        while(zn[rr]>mid){
            rr--;
        }
        while(zn[ll]<mid){
            ll++;
        }
        if(ll<=rr){
            swap(zn[ll],zn[rr]);
            ll++;
            rr--;
        }
    }while(ll<=rr);
    if(aim<=rr) qs(l,rr,aim);
    else if(aim>=ll)
        qs(ll,r,aim);
    else{
        cout<<zn[rr+1];
        exit(0);
    }
}
int main(){
    scanf("%d%d",&n,&Aimee);
    for(int i=1;i<=n;++i){
        scanf("%d",&zn[i]);
    }
    qs(1,n,Aimee+1); 
    return 0;
}