基于快速排序的快速选择

#include <stdio.h>#include <stdlib.h>#include <time.h>#define MAXSIZE100#define CUTOFF 3//#define SELECTNUM24//当要求选特定的第K个时，可使用#definetypedef int elemtype;void Swap(elemtype *,elemtype *);void InsertSort(elemtype *,int);elemtype median3(elemtype *,int,int);void QSelect(elemtype *,int,int,int);void main(){elemtype buffer[MAXSIZE];int i;int selectnum;int low=0;int high=MAXSIZE;srand(time(NULL));for(i=0;i<MAXSIZE;i++)buffer[i]=rand()%(MAXSIZE*10);selectnum=rand()%MAXSIZE;QSelect(buffer,selectnum,low,high);printf("The %d number is %d/n",selectnum,buffer[selectnum]);}void QSelect(elemtype buffer[],int k,int low,int high){elemtype pivot;int i,j;if(high-low+1>CUTOFF){pivot=median3(buffer,low,high);i=low;j=high-1;//MARK!for(;;){while(buffer[++i]<pivot);while(buffer[--j]<pivot);if(i<j)Swap(&buffer[i],&buffer[j]);elsebreak;}//forSwap(&buffer[i],&buffer[high-1]);//枢纽归位if(k<=i)QSelect(buffer,k,low,i-1);else if(k>i+1)QSelect(buffer,k,i+1,high);}//ifelseInsertSort(buffer+low,high-low+1);}elemtype median3(elemtype buffer[],int left,int right){int center=(left+right)/2;if(buffer[left]>buffer[center])Swap(&buffer[left],&buffer[center]);//使中间是三者中的中值!MARK!if(buffer[left]>buffer[right])Swap(&buffer[left],&buffer[right]);if(buffer[right]<buffer[center])Swap(&buffer[right],&buffer[center]);Swap(&buffer[center],&buffer[right-1]);//将枢纽和最后一个元素交换return buffer[right-1];}void Swap(elemtype *st,elemtype *nd){elemtype tmp;tmp=*st;*st=*nd;*nd=tmp;}void InsertSort(elemtype buffer[],int N){int j,p;elemtype tmp;for(p=0;p<N;p++){tmp=buffer[p];for(j=p;j>0&&buffer[j-1]>tmp;j--)buffer[j]=buffer[j-1];buffer[j]=tmp;}//for}

 

有时候就想，就为了一个简简单单的选择而写70多行代码，调用4个函数是否值得……