求第i个小的元素 时间复杂度O(n)

#include<iostream>    //求第i个小的元素 时间复杂度O(n)
#include<cstdlib>
#include<ctime>
using namespace std;

void swap(double *dPara1, double *dPara2)  
{  
    double temp = 0.0;  
    temp = *dPara1;  
    *dPara1 = *dPara2;  
    *dPara2 = temp;  
}  


int randompartitionA( double dArr[], int p, int q)   //划分
{
   srand((unsigned)time(NULL));
   int account = q-p+1;
   int index = 0;
   int i = p;
   index = rand()%account+p;
   swap(dArr[p], dArr[index]);
   int x = dArr[p];
   for( int j=p+1; j<=q; j++)
   {
	   if( dArr[j]<=x )
	   {
		   i++;
		   swap(dArr[i], dArr[j]);
	   }
   }
   swap(dArr[i], dArr[p]);
   return i;
}

int randompartitionB( double dArr[], int p, int q) //划分
{
   srand((unsigned)time(NULL));
   int account = q-p+1;
   int index = 0;
   
   index = rand()%account+p;
   swap(dArr[p], dArr[index]);
   double x = dArr[p];
   
   int low = p;
   int high = q;
   
   while( low<high )
   {
	   while(low<high&&x<dArr[high]) --high;
	   dArr[low]=dArr[high];
	   while(low<high&&x>dArr[low]) ++low;
	   dArr[high]=dArr[low];
   }
   dArr[low] = x;
   return low;
}

double RANDOMIZED_SELECT( double dArr[], int p, int q, int i)
{
    if( p==q )
	{
		return dArr[p];
	}
	int r = randompartitionB( dArr, p, q);  // int r = randompartition( dArr, p, q);
	int k = r-p+1;
	if( i==k )
	{
		return dArr[r];
	}
	else if( i<k )
	{
		return RANDOMIZED_SELECT( dArr, p, r-1, i);
	}
	else
	{
	    return RANDOMIZED_SELECT( dArr, r+1, q, i-k);
    }
}
int main()
{   
   double darr[9] = { 1.0, 2.0 ,6.3, 3.5, 8.3, 0.43, 9, 10, 2.2 };   
   cout<<RANDOMIZED_SELECT( darr, 0, 8, 5);
   cout<<endl;
   return 0;

}