求助，代码看了半天了不知道哪有问题，既然点随机错我猜大概和随机数的选取有关，虽然我也不确定。。。

#include <bits/stdc++.h>
using namespace std;
#define ll long long

const int N = 5e6 + 10;
int a[N], b[N];

int randint(int l, int r){ // 生成在 [l, r] 之间的随机数
	int tmp = rand();
	if(tmp < 0) tmp = -tmp;
	return tmp % (r - l + 1) + l;
}

int find(int l, int r, int k) {
	//if(k < l || k > r) return -114514;
	if(l >= r) return a[l];
	int pos = randint(l, r);
	int pl = l, pr = r;
	for(int i = l; i <= r; i++) {
		if(a[i] < a[pos]) b[pl++] = a[i];
		else if(a[i] > a[pos]) b[pr--] = a[i];
	}
	for(int i = pl; i <= pr; i++) b[i] = a[pos];
	for(int i = l; i <= r; i++) a[i] = b[i];
	//printf("%d   %d %d\n", pos, pl, pr);
	if(pl <= k && k <= pr) return a[pos];
	if(k < pl) return find(l, pl - 1, k);
	if(k > pr) return find(pr + 1, r, k);
	return -114514;
	// 0~(pl-l-1)  (pl-l)~(pr-l)  (pr-l+1)~(r-l)
}

int main() {
	//freopen("test.in", "r", stdin);
	//freopen("test.out", "w", stdout);
	int n, k;
	srand(time(NULL));
	scanf("%d%d", &n, &k);
	for(int i = 1; i <= n; i++) scanf("%d", &a[i]);
	printf("%d\n", find(1, n, k + 1));
	return 0;
}