归并排序模板

void merge_sort(int a[] , int l ,int r){
    if (l == r) return ;
    //划分区间，并且将左区间和右区间归并排好序
    int mid = l + r >> 1;
    merge_sort(a,l,mid);
    merge_sort(a,mid + 1, r);
    //归并两个区间
    int k = 0 , i = l , j = mid + 1;
    while (i <= mid && j <= r){
        if (a[i] <= a[j]) t[k++] = a[i++];
        else t[k++] = a[j++];
    }
    while (i <= mid) t[k++] = a[i++];
    while (j <= r) t[k++] = a[j++];
    //拷贝回原数组
    for(int i = 0, j = l; j <= r ; j++, i ++) a[j] = t[i];
}