快排六问

#include<iostream>
#include<cstdio>
#include<cmath>
#include<cstdlib>
#include<algorithm>
#include<cstring>
#include<string>
#include<queue>
#include<stack>
#include<queue>
#include<iomanip>

using namespace std;
int n,k,ans;
int a[5000010];
long long read(){
	long long x=0,h=1;char ch=getchar();
	while(ch<'0'||ch>'9'){if(ch=='-')h=-1;ch=getchar();}
	while(ch>='0'&&ch<='9'){x=(x<<1)+(x<<3)+(ch^48);ch=getchar();}
	return x*h;
}

void q_sort(int l,int r){
	if(l==r){
		ans=a[l];
		return ;
	}
	int mid=a[(l+r)>>1];
	int i=l,j=r;
	do {
		while(a[i]<mid&&i<=j)i++;
		while(a[j]>mid&&i<=j)j--;
		if(i<=j){
			swap(a[i],a[j]);
			i++,j--;
		}
	}while(i<=j);
	if(k==j+1){
		ans=a[j+1];
		return ;
	}
	else if(k<=j){
		q_sort(l,j);
	}
	else {
		q_sort(i,r);
	}
}

int main(){
//	freopen("","r",stdin);
//	freopen("","w",stdout);
	n=read();k=read()+1;
	for(int i=1;i<=n;i++)a[i]=read();
	
	q_sort(1,n);
	cout<<ans<<endl;
	return 0;
}

void q_sort(int l,int r){
	if(l>=r)return ;
	int mid=a[(l+r)>>1];
	int i=l,j=r;
	do {
		while(a[i]<mid&&i<=j)i++;
		while(a[j]>mid&&i<=j)j--;
		if(i<=j){
			swap(a[i],a[j]);
			i++;j--;
		}
	}while(i<j);
	if(i<r)q_sort(i,r);
	if(j>l)q_sort(l,j);
	return ;
}