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
;

//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;

}