随机化快速排序

#include<iostream>
#include<cstdlib>
#include<ctime>
using namespace std;
const int maxn = 5000;

int main()
{
    void Randomized_QuickSort(int A[], int p, int r);
    void out(int A[], int n);
    int n, A[maxn], B[maxn];
    cin >> n;
    for(int i  = 0; i < n; ++i)
    {
        cin >> A[i];
        B[i] = A[i];
    }
    Randomized_QuickSort(A, 0, n-1);
    out(A, n);
    return 0;
}

void Randomized_QuickSort(int A[], int p, int r)
{
    int Randomized_Partition(int A[], int p, int r);
    if(p < r)
    {
        int q = Randomized_Partition(A, p, r);
        Randomized_QuickSort(A, p, q-1);
        Randomized_QuickSort(A, q+1, r);
    }
}

int Randomized_Partition(int A[], int p, int r)
{
    int Partition(int A[], int p, int r);
    void exchange(int &a, int &b);
    int i = rand()%time(NULL)%(p+1)+r-p;//产生p到r的随机数
    //或者是srand(time(NULL)); rand()%(p+1)+r-p;
    //注：rand()%40产生的是0~39的随机数
    exchange(A[r], A[i]);   //交换两数，产生随机主元
    return Partition(A, p, r);
}

int Partition(int A[], int p, int r)
{
    void exchange(int &a, int &b);
    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]);
        }
    }
    i = i + 1;
    exchange(A[i], A[r]);
    return i;
}

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

void out(int A[], int n)
{
    for(int i  = 0; i < n; ++i)
        cout << A[i] << " ";
    cout << endl;
}

由于快速排序的平均性能较好，为了降低最坏情况的出现概率，提高效率，采用快速排序的随机化方案，用随机函数随即选取主元，使算法趋向于平均情况(nlgn).