Random_Select

#include "stdafx.h"

int PARTITION(int a[], int p, int r)                                           //以遞增順序排序
{
    int x = a[r];
    int i = p - 1;
    for (int j = p; j < r; ++j)
    {
        if (a[j] < x)
        {
            ++i;
            int temp = a[i];
            a[i] = a[j];
            a[j] = temp;
        }
    }
    int temp = a[i + 1];
    a[i + 1] = a[r];
    a[r] = temp;
    return i + 1;
}

int Random_Partition(int a[], int p, int r)
{
    srand(time(NULL));                                            //以當前時間作爲種子，以便每次產生的隨機數不同
    int i = p+rand() % (r - p);                                 //產生隨機數
    int temp = a[i];
    a[i] = a[r];
    a[r] = temp;
    return PARTITION(a, p, r);
}

int Random_Select(int a[], int p, int r, int i)
{
    int q = 0,k = 0;
    if (p==r)
    {
        return a[p];
    }
    else
    {
        q = Random_Partition(a, p, r);
        k = q - p + 1;
        if (i==k)
        {
            return a[q];
        }
        else if (i < k)
        {
            return Random_Select(a, p, q - 1, i);
        }
        else
            return Random_Select(a, q + 1, r, i - k);
    }
}

int _tmain(int argc, _TCHAR* argv[])
{
    int a[] = { 3, 1, 7, 5, 9, 4, 12, 2 };
    int len = sizeof(a) / 4 - 1;
    cout << Random_Select(a, 0, len,7) << endl;
    return 0;
}