算法2.6-2.7快速排序(QuickSort)

这是一个非常经典的排序算法，O(NlogN)阶。

 

#include
#include
#include
#include

#define MAX_LENGTH 100

/*Show usage*/
void usage(char * prog)
{
        printf("%s Usage:\n", prog);
        printf("%s \n", prog);
}

/*Generate and initialize the list*/
int * generate_list(int count)
{

        int i;
        int * list;
        list = (int *)malloc(count*sizeof(int));
        if(list == NULL)
        {
                perror("malloc");
                return NULL;
        }

        /*Initialize the list with integers less than 100*/
        srandom((unsigned int)time(NULL));
        for (i  = 0; i < count ; i ++)
                list[i] = random()%100;

        return list;
}

/*Show the list*/
void show_list(int * list, int length)
{
        int i;
        for(i = 0; i < length; i ++)
                printf("%d      ", list[i]);

        printf("\n");
}

/*algorithm*/

int partition(int * list, int first, int last)
{
        int left = first;
        int right = last;
        int pivot = list[left];
        while(left < right)
        {
                while((right > left) && (list[right] >= pivot))
                        right --;
                if(left < right)
                {
                        list[left] = list[right];
                        left ++;
                }
                while((left < right) && (list[left] < pivot))
                        left ++;
                if(left < right)
                {
                        list[right] = list[left];
                        right --;
                }
        }
        list[left] = pivot;
        return left;
}

void quick_sort(int * list, int first, int last)
{
        if(first < last)
        {
                int split = partition(list, first, last);
                quick_sort(list, first, split-1);
                quick_sort(list, split +1, last);
        }
        return;
}

int main(int argc, char * argv[])
{
        int length;
        int * list = NULL;
        struct timeval tpstart,tpend;
        float timeuse;

        /*Deal with the arguments*/
        if(argc != 2)
        {
                usage(argv[0]);
                exit(127);
        }

        length = atoi(argv[1]);
        if(!length || length > MAX_LENGTH)
        {
                usage(argv[0]);
                exit(129);
        }

        list = generate_list(length);
        if(list == NULL)
                exit(128);
        else
        {
                printf("Soruce list:\n");
                show_list(list, length);

                gettimeofday(&tpstart,NULL);
                quick_sort(list, 0, length -1);
                gettimeofday(&tpend,NULL);
                printf("Ordered list:\n");
                show_list(list, length);
                timeuse=1000000*(tpend.tv_sec-tpstart.tv_sec)+ tpend.tv_usec-tpstart.tv_usec;
                timeuse/=1000;
                printf("time used: %f msec\n", timeuse);
        }

        free(list);
        return 1;

}