线性时间选择第K小元素（随机化选择第K小元素）C++

 

// 随机化产生第K小数.cpp : Defines the entry point for the console application.
//

#include "stdafx.h"
#include<iostream>
#include<ctime>
using namespace std;
#define N 100
double a[N];

//产生数组的随机下标，以其对应的数组元素作为比较关键字
int random(int m,int n)
{
 //以系统时间作为随机种子
    srand((unsigned)time(NULL));
  return m + rand()%(n-m+1);
}
//一趟快速排序
int qartition(double *a,int begin,int end)  
{
 double x = a[end];
 int i = begin-1,j=begin;
 while(j<end)
 {
       if(a[j]<=x)
     {
       i++;
       double temp = a[j];
       a[j]=a[i];
       a[i]=temp;
     }
     j++;
 }
 double temp = a[end];
 a[end] = a[i+1];
 a[i+1]=temp;
 return i+1;
}

//一趟随机化快速排序

int random_qartition(double *a,int begin,int end)
{
 int k = random(begin,end);
 //cout<<"random k="<<k<<endl;
 double temp = a[k];
 a[k]=a[end];
 a[end]=temp;
 return qartition(a,begin,end);
}
//随机化选择第i小元素
double random_K_select(double *a,int begin,int end,int i)
{
 if(begin>end)
  return 65535;
 else if(begin==end)
  return a[begin];
 else
 {   
    int k = random_qartition(a,begin,end);
    if(i-1==k)
        return a[k];
    else if(i-1<k)
        return random_K_select(a,begin,k-1,i);
    else  return random_K_select(a,k+1,end,i);
 }
}

int _tmain(int argc, _TCHAR* argv[])
{
        memset(a,0.0,sizeof(a));
        int n;
        cout<<"请输入这些元素的个数："<<endl;
        cin>>n;
        cout<<"请输入这些元素："<<endl;
        int i = 0;
        for(i=0;i<n;i++)
        {
              cin>>a[i];
        }
    cout<<"请输入需要查询的第K小元素的K值(以-1结束)："<<endl;
    int k;
    //double key;
    cin>>k;
    while(k!=-1)
    {      if(k>n)
           {
               cout<<"没有这个元素，因为输入的k值不能大于n："<<n<<"，请重新输入k值"<<endl;
          cin>>k;
          continue;
            }
          //key = random_K_select(a,0,n-1,k);
          cout<<"第 "<<k <<" 小元素是："<<endl;
        cout<<random_K_select(a,0,n-1,k)<<endl;
       cin>>k;
    }
   system("pause");
 return 0;
}