快速排序的优化及其与qsort的比较

      快速排序是用的最多的一种排序方法，《编程珠玑》在第十一章《排序》中讨论了快速排序的几个版本，其实版本的不同，都是体现在partition函数上。

      partition版本1,其实就是《算法导论》给出的版本，很好理解；

     

int partition1(int a[], int begin, int end)
{
int pivotkey = a[end];
int i = begin;
int j = begin - 1;
int tmp;
for (i = begin; i < end; i ++)
{
if (a[i] <= pivotkey)
{
j ++;
tmp = a[j];
a[j] = a[i];
a[i] = tmp;
}
}
j ++;
tmp = a[j];
a[j] = a[end];
a[end] = tmp;
return j;
}

     

     partition版本2：其实本科的时候最先学习快速排序，就是使用的这个版本。后来看到版本1，觉得版本1更好理解。但是该版本的效率更高，使用了两个指针：

       

int partition2(int a[],int begin, int end)
{
int pivotkey = a[begin];
int tmp;
int low = begin;
int high = end;
while (low < high)
{
while (low < high && a[high] >= pivotkey )
--high;
tmp = a[low];
a[low] = a[high];
a[high] = tmp;
while(low < high && a[low] <= pivotkey)
++low;
tmp = a[low];
a[low] = a[high];
a[high] = tmp;
}
return low;
}

        partition 版本3:对版本2的优化：其实不用每次交换a[low]和a[high]，直接覆盖好了，类似于前面的插入排序的改进

     

int partition3(int a[],int begin, int end)
{
int pivotkey = a[begin];
int low = begin;
int high = end;
while (low < high)
{
while (low < high && a[high] >= pivotkey )
--high;
a[low] = a[high];
while(low < high && a[low] <= pivotkey)
++low;
a[high] = a[low];
}
a[low] = pivotkey;
return low;
}

测试程序及其结果

# include <stdio.h>
# include <stdlib.h>
# include <time.h>
# include "chapter11.h"
# include "chapter12.h"
# define N 500000
int cmp(const void* a,const void* b)
{
return *(int *)a - *(int *)(b);
}
int main()
{
//	testchapter12
//	int m = 10, n = 100;
//	genknuth(m, n);
//	genbyme(m,n);
//	int a[] = {5,7,6,4,2,8,6};
//	int a[] = {6,4,10,12,7,8};
int a
;
int i;
for(i = 0; i < N; i ++)
a[i] = rand()%(10 * N);
int len = sizeof (a)/ sizeof(a[0]);
clock_t start = clock();
//	quickSort1(a, 0, len - 1);
//	quickSort2(a, 0, len - 1);
//	quickSort3(a, 0, len - 1);
qsort(a,len,sizeof(a[0]),cmp);
//	partition3(a,0,len - 1);
clock_t end = clock();
printf("the cost time is %ld", (long int)end - start);
return 0;
}

结果：

版本1：the cost time is 141

版本2：the cost time is 125

版本3：the cost time is 110

qsort:  the cost time is 94