H面试程序（14）： 快速排序法

本文代码借鉴了http://blog.csdn.net/fduan/article/details/7914331

但也做了一些小的优化，就是在选择枢纽元的时候，采用了三数取中法，避免取到最大或最小的数作为枢纽元，在待排数比较多的情况下能有效减少排序的时间

 

#include <stdio.h>#include <time.h>void swap(int *a, int *b){int temp;temp = *a;*a = *b;*b = temp;}void disp_array( int a[], int n ){    int i;    for( i = 0; i < n; ++i )        printf( "%d ", a[i] );    printf( "\n" );}int partition( int a[], int low, int high ){  int m = low + (high - low) / 2;  //采用三数取中的方法，尽量避免枢纽元素是最大数或最小数if(a[low] >a[high])swap(&(a[low]),&(a[high]));if(a[m] > a[high])swap(&(a[m]), &(a[high]));    if(a[m] > a[low])swap(&(a[m]), &(a[low]));    int piv_val = a[low];    while( low < high )    {        while( low < high && a[high] >= piv_val ) --high;        a[low] = a[high];        while( low < high && a[low] <= piv_val ) ++low;        a[high] = a[low];    }    a[low] = piv_val;    return low;}void qsort_recur( int a[], int low, int high ){    int idx_piv;    if( low < high )    {        idx_piv = partition( a, low, high );        qsort_recur( a, low, idx_piv - 1 );        qsort_recur( a, idx_piv + 1, high );    }}void quick_sort( int a[], int n ){    qsort_recur( a, 0, n - 1 );}int main(){   clock_t start, finish;start = clock();    int a[] = {  10,9,8,7,6,5,4,3,2,1};    int n = sizeof(a)/sizeof(int);        printf( "Before sorting: " );    disp_array( a, n );    quick_sort( a, n );    printf( "After sorting: " );    disp_array( a, n );    finish = clock();    printf("\n本次计算一共耗时: %f秒\n\n", (double)(finish-start)/CLOCKS_PER_SEC);    return 0;}