救救孩子吧，2WA，2RE，1TLE

#include<cstdio>
#include<cctype>
#include<cmath>
#include<cstring>
#include<iostream>
#include<algorithm>
#include<climits>
#include<map>
using namespace std;

const int maxn = 5000000;
int randPartition(int A[], int left, int right)
{
    int p = (round(1.0 * rand() / RAND_MAX * (right - left) + left));
    swap(A[p],A[left]);
    int temp = A[left];
    while (left < right)
    {
        while (left<right && A[right]>temp)
        {
            right--;
        }
        A[left] = A[right];
        while (left < right && A[left] <= temp)
        {
            left++;
        }
        A[right] = A[left];
    }
    A[left] = temp;
    return left;
}
void quickSort(int A[],int left,int right)
{
    if (left<right)
    {
        int pos = randPartition(A, left, right);
        quickSort(A, left, pos - 1);
        quickSort(A, pos + 1, right);
    }

}

int randSelect(int A[],int left,int right,int K)
{
    if (left==right)
    {
        return A[left];
    }
    int p = randPartition(A, left, right);
    int M = p - left + 1;
    //int M = p - left;
    if (K==M)
    {
        return A[p];
    }
    else if (K<M)
    {
        return randSelect(A, left, p - 1, K);
    }
    else
    {
        return randSelect(A, p + 1, right, K - M);
    }
 }

int nums[maxn];

int main() {
    int n,k;
    scanf("%d%d", &n, &k);
    for (int i = 0; i < n; i++)
    {
        int val;
        cin >> val;
        nums[i] = val;
    }
    int pos = randSelect(nums, 0, n - 1, k+1);
    printf("%d", nums[pos-1]);
    return 0;
}