786. 第k个数

#include <iostream>
using namespace std;

const int N = 1e6 + 10;

int n, k;
int q[N];

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

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