求助！AC 60%

#include<iostream>
#include<algorithm>
#include<ctime>
using namespace std;
const int N=5000100;
int n,m,a[N];
int qs(int i,int j){
    srand(time(0));
    int key=a[(rand()%(j-i+1))+i];
    while(i<j){
        while(i<j&& a[j]>=key) j--;
        if(i<j) a[i]=a[j];
        while(i<j&& a[i]<=key) i++;
        if(i<j) a[j]=a[i];
    }
    a[i]=key;
    return i;
}
int find(int left, int right, int k)
{   
    int tem=qs(left,right);
    if(k==tem) printf("%d",a[k]);
    else if(k-1<tem) find(left,tem-1,k);
    else find(tem+1,right,k);
    return 0;
}
int main(){
    cin>>n>>m;
    for(int i=0;i<n;i++) scanf("%d",&a[i]);
    find(0,n-1,m);
    return 0;
}
/*
5 1
4 3 2 1 5
---------
2
*/