oh 实在没看懂我这为什么不对/(ㄒoㄒ)/~~ 求大佬帮帮忙

#include <string.h>
#define ll long long
using namespace std;

int n; //n up to 500,0000; and n is a 奇数;
int k;
int arr[5000000];

void quicksort
  (int arr[], int l, int r)
{
    if (l >= r) return;
    int i = l, j = r, x = arr[(l + r) >> 1];
    while (i < j) {
        while (arr[i] < x) i++;
        while (arr[j] > x) j--;
        if (i < j)
        {
            swap(arr[i], arr[j]);
            i++;
            j--;
        }
    }
    if (k <= j)
        quicksort(arr, l, j);
    else if (k > i)
        quicksort(arr, i, r);
    else
    {
        cout << arr[i] << endl;
        return;
    }
}
int main()
{
    cin >> n >> k;
    for (int i = 0; i < n; i++)
        cin >> arr[i];
    quicksort(arr, 0, n - 1);
    return 0;
}