第四次：第二题：merge_sort

```c
#include <stdio.h>

void merge_sort(int arr[], int p, int q);
void merge(int arr[], int p, int mid, int q);
void show_sort(int s[], int n);

int main(){
    int i = 0;
    int arr[8] = { 7,5,2,4,1,6,3,0 };
    merge_sort(arr, 1, 8);
    printf("result : \n");
    show_sort(arr, 8);
    return 0;
}

void merge_sort(int arr[], int p, int q){
    int mid;
    if (arr == NULL || p > q || p == q) {
        return ;
    }
    mid = (p + q) / 2;
    merge_sort(arr, p, mid);
    merge_sort(arr, mid + 1, q);
    merge(arr, p, mid, q);
}

void merge(int arr[], int p, int mid, int q){
    int i,j,k;
    int leftarr[100], rightarr[100];
    int numL = mid - p + 1;
    int numR = q - mid;
    for (i = 0; i < numL; i++){
        leftarr[i] = arr[p - 1 + i];
    }
    leftarr[i] = 2147483647;  // max_int
    for (i = 0; i < numR; i++){
        rightarr[i] = arr[mid + i];
    }
    rightarr[i] = 2147483647;  // max_int
    i = 0;
    j = 0;
    for (k = p; k <= q; k++){
        if(leftarr[i] <= rightarr[j]){
            arr[k - 1] = leftarr[i];
            i++;
        }
        else{
            arr[k - 1] = rightarr[j];
            j++;
        }
    }
    show_sort(arr, 8);
}

void show_sort(int s[], int n){
    int i;
    for(i = 0; i < n; i++){
        printf("%3d  ", s[i]);
    }
    printf("\n");
}
```