封裝成函數(shù)merge_sort_up_to_down（自頂向下方式）和merge_sort_down_to_up（自底向上方式）：

#include <malloc.h>

//歸并操作
void merge(int *a,int *aux,int lo,int mid,int hi)
{
    int i,j,k;
    for(k=lo; k<=hi; k++)
    {
        aux[k] = a[k];
    }
    i = lo;
    j = mid+1;
    for(k=lo; k<=hi; k++)
    {
        if(i>mid)
        {
            a[k] = aux[j++];
        }
        else if(j>hi)
        {
            a[k] = aux[i++];
        }
        else if(aux[j]<aux[i])
        {
            a[k] = aux[j++];
        }
        else
        {
            a[k] = aux[i++];
        }
    }
}

void sort(int *a,int *aux,int lo,int hi)
{
    int mid;
    if(lo>=hi)
    {
        return;
    }
    mid = (lo+hi)>>1;
    sort(a,aux,lo,mid);
    sort(a,aux,mid+1,hi);
    merge(a,aux,lo,mid,hi);
}

//自頂向下歸并排序
void merge_sort_up_to_down(int *a,int len)
{
    int *aux = (int*)malloc(len*sizeof(int));
    sort(a,aux,0,len-1);
}

int min(int i,int j)
{
    if (j < i)
    {
        return j;
    }
    return i;
}

//自底向上歸并排序
void merge_sort_down_to_up(int *a,int len)
{
    int *aux = (int*)malloc(len*sizeof(int));
    int sz,lo;
    for (sz = 1; sz < len; sz *= 2)
    {
        for (lo = 0; lo < len-sz; lo += 2 * sz)
        {
            merge(a, aux, lo, lo+sz-1, min(lo+2*sz-1, len-1));
        }
    }
}

測試：

#include <stdio.h>

//打印輸出數(shù)組
void print_arr(int *a,int len)
{
    int i;
    if(len<1)  //數(shù)組長度必須大于0
    {
        printf("length greater than 0");
        return;
    }
    //打印整個數(shù)組
    printf("[");
    for(i=0; i<len-1; i++)
    {
        printf("%d ",a[i]);
    }
    printf("%d]\n",a[len-1]);
}

int main()
{
    int a[] = {9,0,6,5,8,2,1,7,4,3};
    int len = sizeof(a)/sizeof(int);
    print_arr(a,len);
    merge_sort_up_to_down(a,len);
    //merge_sort_down_to_up(a,len);
    print_arr(a,len);
    return 0;
}

輸出：
[9 0 6 5 8 2 1 7 4 3]
[0 1 2 3 4 5 6 7 8 9]