想链表中插入结点，为什么函数声明用的是**H而不是*H？

#include <stdlib.h>
#include <stdio.h>
#define LEN sizeof(struct node)
struct node
{    int Data;
    struct node *Link;
} *H;
void InsertNode(struct node **H, struct node *p, int b)
{ struct node *x;
  x = (struct node *)malloc(sizeof(struct node));  
        /* 取空结点 */
  x->Data = b;      x->Link = 0;  /* 值为b */
  if (p == 0) 
    { x->Link = *H;  *H = x;} /* 表头插入 */
  else
    { x->Link = p->Link;  p->Link = x;}  /* p后插入 */
}
int main()
{  int A[8]={25,73,60,37,98,90,24,30};
   int i;
   struct node *p;
   H = (struct node *)malloc(sizeof(struct node));
   H->Data = A[0];  H->Link = 0;
   p = H;
   for (i=1; i<8; i++)
   {   p->Link = (struct node *)malloc(sizeof(struct node));
       p=p->Link;  p->Data = A[i];   p->Link = 0;
   }   
   p = H;
   printf("打印结点中元素值：\n");
   while (p!=0)
   {  printf("%5d",p->Data);        p = p->Link;   }   
   printf("\n");
   p=0;
   InsertNode(&H, p, 15);
   printf("打印新表中结点元素值：\n");
   p=H;
   while (p!=0)
   {  printf("%5d",p->Data);        p = p->Link;   }   
   printf("\n");

} 

原函数中使用到H的语句是{ x->Link = *H; *H = x;}，如果H是struct node *H，*H就是struct node,既不能通过x->Link = *H赋值（类型不匹配），也不能用*H = x保存x。
其实也应该可以写成一次指针

void InsertNode(struct node *H, struct node *p, int b)
{ struct node *x;
  x = (struct node *)malloc(sizeof(struct node));  
        /* 取空结点 */
  x->Data = b;      x->Link = 0;  /* 值为b */
  if (p == 0) 
    { x->Link = H;  H = x;} /* 表头插入 */
  else
    { x->Link = p->Link;  p->Link = x;}  /* p后插入 */
}

x = (struct node *)malloc(sizeof(struct node)); 
改为
x = (struct node)malloc(sizeof(struct node)); 

*H 是node指针
**H 是指向 node指针的指针