算法（三）：归并排序

归并排序同为分治法，但是与快速排序不同

快速排序更偏向于分，在分的递归过程中的两侧数据交换

而归并排序更偏向于合并，在两半部分合并的递归中的按顺序排序

代码：

root@ubuntu:/mnt/shared/appbox/mgsort# cat mgsort.c
#include <stdlib.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <stdio.h>

static int compare_int(const void *int1, const void *int2)
{
        if(*(const int *)int1 == *(const int *)int2)
                return 0;

        return *(const int *)int1 > *(const int *)int2 ? 1 : -1;
}

static int merge(void *data, int esize, int i, int j, int k, int (*compare)(const void *key1, const void *key2))
{
        char *a = data;
        char *m;
        int ipos, jpos, mpos;

        ipos = i;
        jpos = j + 1;
        mpos = 0;

        if((m = (char *)malloc(esize * ((k-i)+1))) == NULL)
                return -1;

        while(ipos <= j || jpos <= k)
        {
                if(ipos > j)// left no more elements to merge
                {
                        while(jpos <= k)//merge right
                        {
                                memcpy(&m[mpos * esize], &a[jpos * esize], esize);
                                jpos++;
                                mpos++;
                        }
                        continue;
                }
                else if(jpos > k)//right no more elements to merge
                {
                        while(ipos <= j)// merge left
                        {
                                memcpy(&m[mpos * esize], &a[ipos * esize], esize);
                                ipos++;
                                mpos++;
                        }
                        continue;
                }

                //both left and right need merge
                if(compare(&a[ipos * esize], &a[jpos * esize]) < 0)
                {
                                memcpy(&m[mpos * esize], &a[ipos * esize], esize);
                                ipos++;
                                mpos++;
                }
                else
                {
                                memcpy(&m[mpos * esize], &a[jpos * esize], esize);
                                jpos++;
                                mpos++;
                }
        }

        memcpy(&a[i * esize], m, esize * ((k-i)+1));
        free(m);

        return 0;
}

int mgsort(void *data, int size, int esize, int i, int k , int (*compare)(const void *key1, const void *key2))
{
        int j;

        if(i < k)
        {
                j = (int)(((i + k -1))/2);

                if(mgsort(data, size, esize, i, j, compare) < 0)
                        return -1;

                if(mgsort(data, size, esize, j+1, k, compare) < 0)
                        return -1;

                if(merge(data, esize, i, j, k, compare)< 0)
                        return -1;
        }
        return 0;
}

int main(int argc, char *argv[])
{
        int ret;
        int i, j, k;
        i = 11;
        j = 12;
        int array[10]= {1,4,7,2,5,8,3,6,9,10};

        printf("\n******************compare_init test**********************\n");
        printf("compare_int(&i, &j)=%d\n", compare_int(&i, &j));
        printf("compare_int(&j, &i)=%d\n", compare_int(&j, &i));
        printf("compare_int(&i, &i)=%d\n", compare_int(&i, &i));
        printf("\n******************partition    test**********************\n");

        mgsort(array, 10, sizeof(int), 0, 9, compare_int);


        for(i=0; i<10; i++)
                printf("%d ", array[i]);
        printf("\n");


        return 0;
}

运行结果：

root@ubuntu:/mnt/shared/appbox/mgsort# ./mgsort 

******************compare_init test**********************
compare_int(&i, &j)=-1
compare_int(&j, &i)=1
compare_int(&i, &i)=0

******************partition    test**********************
1 2 3 4 5 6 7 8 9 10 
root@ubuntu:/mnt/shared/appbox/mgsort#