算法导论 9.3-6 k分位数

1.第三版的算法导论上面的翻译感觉略有问题，“k分位数是指 能把有序集合分成k个等大小集合的第k-1个顺序统计量”。 觉得应该改为， “k分位数是指 能把有序集合分成k个等大小集合的k-1个顺序统计量”。

2.寻找k分位数，利用二分思想，寻找k分位数，首先找其中的第k/2分位点，一次递归寻找需要的点。、

#include <iostream>
#include <cstdlib>
#include <cstdio>
#include <ctime>
#include <algorithm>
using namespace std;

int* createarray(int);
void myswap(int*A,int p,int q);
void print(int*,int,int);
int random_select(int*,int,int,int);
int random_partition(int*A,int,int);
int mypartition(int*,int,int);
int *kth_qua(int *A,int p,int r,int k,int *,int t);

int main ()
{
    int *A = createarray(16);
   // srand(time(NULL));
    print(A,0,16);
    int k = 4;
    int *B = new int[4];
    kth_qua(A,0,15,4,B,4);

    print(B,0,4);
    sort(A,A+16);
    print(A,0,16);
}
int* createarray(int n)
{
    int *aret = new int[n];
    srand(time(NULL));
    for (int i = 0;i < n;i ++)
    {
        *(aret+i) = rand()%1024;
    }
    return aret;
}

void myswap(int *A,int p,int q)
{
    int temp = A[p];
    A[p] = A[q];
    A[q] = temp;
    return;
}
void print(int *A,int p,int q)
{
    for (int i = p;i < q;i ++)
    {
        cout << A[i] << '\t' ;
    }
    cout << "\n----------------endl\n";
}

int mypartition(int*A,int p,int r)
{
    int i = p -1;
    int j = p;
    int key = A[r];
    for (j = p;j <=r-1; j ++)
    {
        if (A[j] < key)
        {
            i ++;
            myswap(A,i,j);
        }
    }
    i ++;
    myswap(A,i,r);
    return i;
}

int random_partition(int *A,int p,int r)
{
    int i = rand()%(r-p+1) + p;
   // cout << "e " << p << ' ' << r << ' '<< endl;
    myswap(A,i,r);
    return mypartition(A,p,r);
}

int random_select(int *A,int p,int r,int k)
{
    int q = random_partition(A,p,r);
    cout << "rr " << p <<' '<< r <<' '<<  k << endl;
    int i = p,j = 0;
    int t = q-i+1;
    while (t != k)
    {
       // cout << "dsa " << k  << ' ' << t <<' '<< q << ' ' <<i << ' ' << j  <<endl;
        if (t < k)
        {
            i =  q +1;
            k = k-t;
        }
        else
        {
            j = q-1;
        }
        q = random_partition(A,i,j);
        t =q-i+1;
    }
    return q;
}

int *kth_qua(int *A,int p,int r,int k,int *B,int t)
{
    if ((r-p+1) <= t)
    {
        return 0;
    }
    int sub_k = (r-p+1)/t;
    int mid_k = sub_k/2;
    int dmin_k = mid_k*t;
    int s  = random_select(A,p,r,dmin_k);
   // cout <<"s " <<  s << endl;
  //  print(A,0,16);
    //cout << "dsad " << p << ' ' << r <<' ' <<s <<endl;
    B[s/t] = A[s];
    kth_qua(A,p,s,k,B,t);
    kth_qua(A,s+1,r,k,B,t);
}