快速选择

#include <iostream>

using namespace std;

const int N = 100010;

int n, k;
int q[N];

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

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

    return 0;
}