算法2.11--合并排序算法的改进算法(MergeSort2)

基本的思路是先从list中依次找出两个有序的子list,merge之，再继续找．scan一轮后，进行下一轮的查找，直到发现从list首到list尾为最大的有序子列(即list有序)为止．

额外的空间开销大，但是充分利用了list中的有序部分，减少了扫描次数．

ＢＴＷ：据说这样的方法适合于并行计算．:)

 

#include
#include
#include

#define MAX_LENGTH 100

/*Show usage*/
void usage(char * prog)
{
        printf("%s Usage:\n", prog);
        printf("%s (should be less than 100)>\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");
}

void merge(int * srclist, int first, int mid, int last)
{
        int * ord_list = (int *)malloc((last - first + 1)* sizeof(int));
        int i = first, j = mid+1, k = 0;
        int t;
        while((i <= mid) && (j <= last))
        {
                if(srclist[i] < srclist[j])
                {
                        ord_list[k] = srclist[i];
                        i ++;
                        k ++;
                }
                else
                {
                        ord_list[k] = srclist[j];
                        j ++;
                        k ++;
                }
        }

        if(i > mid)
        {
                for(t = j; t <= last; t ++)
                {
                        ord_list[k] = srclist[t];
                        k ++;
                }
        }
        else
        {
                for(t = i; t <= mid ; t ++)
                {
                        ord_list[k] = srclist[t];
                        k ++;
                }
        }

        t = first;
        while(t <= last)
        {
                srclist[t] = ord_list[t - first];
                t ++;
        }
        free(ord_list);
}

/*algorithm*/
void merge_sort2(int * list, int length)
{
        while(length > 1)
        {
                int i = 0;
                int j, k;
                do
                {
                        for(j = i; (j < length-1)&&(list[j] <= list[j+1]); j ++);

                        if(j == length-1)
                        {
                                if(i == 0)
                                {
                                        return;
                                }
                                else
                                {
                                        break;
                                }
                        }

                        for(k = j +1; (k < length-1) && (list[k] <= list[k+1]); k ++);
                        merge(list, i, j, k);
                        i = k + 1;
                }while(i <= length-1);
        }
}

int main(int argc, char * argv[])
{
        int length;
        int * list = NULL;

        /*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);

        printf("Source List:\n");
        show_list(list, length);

        merge_sort2(list, length);

        printf("Ordered List:\n");
        show_list(list, length);

        free(list);
        return 1;
}