20分，大佬帮忙看看

#include<stdio.h>
int n, k;
int a[5000001];
void swap(int *x, int *y)
{
	int tmp = *x;
	*x = *y;
	*y = tmp;
}
void px(int begin,int end)
{
	if (begin >= end)
	{
		return;
	}
	int right = end;
	int left = begin;
	int bj = left;
	while (left < right)
	{
		while (left<right&&a[right] >= a[bj])
		{
			right--;
		}
		while (left<right&&a[left] < a[bj])
		{
			left++;
		}
		swap(&a[left], &a[right]);
	}
	swap(&a[bj], &a[left]);
	if (k < bj)
	{
		px(begin, bj - 1);
	}
	else if (k > bj)
	{
		px(bj + 1, end);
	}
}
int main()
{
	scanf("%d%d", &n, &k);
	for (int i = 0; i < n; i++)
	{
		scanf("%d", &a[i]);
	}
	px(0, n-1);
	printf("%d", a[k]);
	return 0;
}