快速排序（取中位数法）

#include<iostream>
#include<algorithm>
#include<iterator>
using namespace std;

// 求首元素、中间元素和尾元素的中位数，将中位数与首元素交换位置
inline void medianAsPivot(int arr[], const int& left, const int& right)
{
    const int middle = left+(right-left)>>1;
    const int l = arr[left];
    const int m = arr[middle];
    const int r = arr[right];

    if(l<=m&&m<=r || r<=m&&m<=l) swap(arr[left], arr[middle]);
    if(l<=r&&r<=m || m<=r&&r<=l) swap(arr[left], arr[right]);   
}

// 求分区位置i，该位置左边的所有元素小于该元素，右边的所有元素不小于该元素
// 同时，求得的这个位置的元素值为第n-i大元素
int partition(int arr[], const int& left, const int& right)
{
    if(left < right)
    {
        medianAsPivot(arr, left, right);
        int i = left;
        int j = right;
        int key = arr[i]; // 首元素作为pivot
        while(i < j)
        {
            while(i<j && arr[j]>=key) j--;
            if(i < j) arr[i++] = arr[j];
            while(i<j && arr[i]<key) i++;
            if(i < j) arr[j--] = arr[i];
        }
        arr[i] = key;
        return i;
    }
    return left;
}

// 快速排序递归实现
void quickSort(int arr[], const int& left, const int& right)
{
    if(left < right)
    {
        int p = partition(arr, left, right);
        quickSort(arr, left, p-1);
        quickSort(arr, p+1, right);
    }
}

int main()
{
    int arr[] = {6, 8, 9, 2, 1, 4, 3, 2, 4, 7, 0, 5, 6};
    int size = sizeof(arr)/sizeof(int);
    copy(arr, arr+size, ostream_iterator<int>(cout, " "));
    cout<<endl;
    
    quickSort(arr, 0, size-1); 
    copy(arr, arr+size, ostream_iterator<int>(cout, " "));
    cout<<endl;

    random_shuffle(arr, arr+size);
    copy(arr, arr+size, ostream_iterator<int>(cout, " "));
    cout<<endl;

    quickSort(arr, 0, size-1); 
    copy(arr, arr+size, ostream_iterator<int>(cout, " "));
    cout<<endl;
    return 0;
}