快速排序（取中位数法）

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#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;}
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