选出数组中指定小的元素

#include <iostream>
#include <time.h>
#include <stdlib.h>
using namespace std;

int random(int a,int b)
{
    srand(NULL);
    return rand()%(b-a)+a;
}

void exchange(int &a,int &b)
{
    int temp;
    temp=a;
    a=b;
    b=temp;
}

int partition(int *a,int p,int r)
{
    int x=a[r];
    int i=p-1;
    for (int j=p;j<r;j++)
    {
        if (a[j]<=x)
        {
            i=i+1;
            exchange(a[i],a[j]);
        }
    }
    exchange(a[i+1],a[r]);
    return i+1;
}

int randomized_partition(int *a,int p,int r)
{
    int i=random(p,r);
    exchange(a[r],a[i]);
    return partition(a,p,r);
}

int randomized_select(int *a,int p,int r,int i)    //选出数组a[p...r]中倒数第i个小的元素
{
    if (p==r)
        return a[p];
    int q=randomized_partition(a,p,r);
    int k=q-p+1;
    if (i==k)
        return a[q];
    else if(i<k)
        return randomized_select(a,p,q-1,i);
    else
        return randomized_select(a,q+1,r,i-k);
}

int main()
{
    int a[]={4,1,3,2,16,30,0,7,5,8,98,34};
    int length=sizeof(a)/sizeof(int)-1;
    cout<<randomized_select(a,0,length,9)<<endl;
    system("pause");
    return 0;
}