归并排序（MERGE-SORT）是建立在归并操作上的一种有效的排序算法,该算法是采用分治法（Divide and Conquer）的一个非常典型的应用。将已有序的子序列合并，得到完全有序的序列；即先使每个子序列有序，再使子序列段间有序。若将两个有序表合并成一个有序表，称为二路归并。

#include<stdio.h>
#include<Windows.h>
#include<assert.h>
void Merge(int *a, int left, int key, int right)
{
    int *tmp = (int*)malloc((right - left + 1) * sizeof(int));
    int i = left;
    int j = key + 1;
    int k = 0;
    while (i <= key && j <= right)
    {
        if (a[i] < a[j])
            tmp[k++] = a[i++];
        else
            tmp[k++] = a[j++];
    }
    while (i <= key)
    {
        tmp[k++] = a[i++];
    }
    while (j <= right)
    {
        tmp[k++] = a[j++];
    }
    for (i = 0; i < k; i++)
    {
        a[left + i] = tmp[i];
    }
    free(tmp);
}
void MergeSort(int *a,int left,int right)
{
    if (a == NULL || left >= right)
    {
        return;
    }
    int key = left + (right - left) / 2;
    MergeSort(a, left, key);
    MergeSort(a, key + 1, right);
    Merge(a, left, key, right);
}
int main()
{

    int a[] = { 21,12,15,32,17,55,64,58,91 };
    int n = sizeof(a) / sizeof(a[0]);
    MergeSort(a,0,n-1);
    for (int i = 0; i < n; i++)
    {
        printf("%d ", a[i]);
    }
    system("pause");
    return 0;
}