核心就是用兩個子數組記錄分割后的兩個數組中的變量， 然后依次比較大小即可。
這里有個細節需要注意一下，
arr_temp1[mid + 1 - low] = INT_MAX;
這段代碼，用來設置一個哨兵， 用這種方法可以避免判斷數組是否為空了

具體的算法的偽代碼可以參考《算法導論》 Chapter 2 算法基礎， P17

源代碼如下：

// =====================【歸并排序】==================

include <stdio.h>
include <stdlib.h>
include <time.h>
include <math.h>
define NUM 20
int arr[NUM] = { 0 };
int arr_temp1[NUM] = { 0 };
int arr_temp2[NUM] = { 0 };
void init()
{
    time_t tm;
    time(&tm);
    srand((unsigned int)tm);

int max_item = 100;
for (int i = 0; i != NUM; i++)
    arr[i] = rand() % max_item;

}
void display(int * arr)
{
    for (int i = 0; i != NUM; i++)
        printf("%-10d", arr[i]);
    printf("\n");
}
void merge(int low, int mid, int high)
{
    for (int ii = 0; ii != mid + 1 - low; ii++)
    {
        arr_temp1[ii] = arr[low + ii];
    }
    arr_temp1[mid + 1 - low] = INT_MAX;

for (int ii = 0; ii != high - mid; ii++)
{
    arr_temp2[ii] = arr[mid + 1 + ii];
}
arr_temp2[high - mid] = INT_MAX;

int i = 0;
int j = 0;
for (int k = low; k != high + 1; k++)
{       
    if (arr_temp1[i] < arr_temp2[j])
        arr[k] = arr_temp1[i++];
    else
        arr[k] = arr_temp2[j++];
}

}
void mergeSort(int low, int high)
{
    if (low >= high)
        return;

int mid = (low + high) / 2;
mergeSort(low, mid);
mergeSort(mid + 1, high);
merge(low, mid, high);

}
void main()
{
    init();
    printf("歸并排序前\n");
    display(arr);

mergeSort(0, NUM - 1);
printf("歸并排序后\n");
display(arr);

}</pre>