更好理解的做法，用下标来做

#include <iostream>
using namespace std;

const int N = 100010;
int n, q[N], k;

int quick_sort(int l, int r, int k)
{
    if (l == r) return q[l];
    int i = l - 1, j = r + 1, x = q[l + r >> 1];
    while (i < j)
    {
        while (q[++ i] < x);
        while (q[-- j] > x);
        if (i < j) swap(q[i], q[j]);
    }
    if (k <= j) quick_sort(l, j, k);
    else quick_sort(j + 1, r, k);
}

int main()
{
    cin >> n >> k;
    for (int i = 0; i < n; i ++)
    {
        scanf("%d", &q[i]);
    }
    cout << quick_sort(0, n - 1, k - 1) << endl;
    return 0;
}