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;
}