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;
}