> > william wrote:
> > > When implementing Linked list, stack, or trees, we always use pointers
> > > to 'link' the nodes.
> > > And every node is always defined as:
> > > struct node
> > > {
> > >   type data; //data this node contains
> > >   ...
> > >   node * nPtr; //the next node's pointer
> > > }

> > > And we also define operations as insert, delete, etc. on the data
> > > structure.
> > > MY QUESTION IS:
> > > why we always pass node**(pointer to the pointer to the node) as
> > > argument to these operations? Why we can not just use the node *?

> > > Thank you!

> > > Ji

> > In a good implementation of a linked list, the nodes are completely
> > hidden. You would never pass node* or node** to any operations.

> What does it mean by nodes are completely hidden(as being refered to,
> nodes means the pointers to the actual struct block in memory, or the
> structs themselves?)

> > What you are looking at is a bad implementation of a linked list. I
> > can't tell you why it uses node** or node* without seeing the code. It
> > shouldn't use either.

> > john

> I paste the sample code below, which came from the book 'C how to
> program'. John, could you please offer me a good example of
> implementation of linked list? Thank you.

> **********************************************************************
> #include <stdio.h>
> #include <stdlib.h>

> struct listNode
> {
>   char data;
>   struct listNode *nextPtr;

> };

> typedef struct listNode ListNode;
> typedef ListNode *ListNodePtr;

> void insert(ListNodePtr *, char);
> char delete(ListNodePtr *, char);
> int isEmpty(ListNodePtr);
> void printList(ListNodePtr);
> void instructions(void);

> int main()
> {
>   ListNodePtr startPtr=NULL;
>   int choice;
>   char item;

>   instructions(); //display the menu;
>   printf(">");
>   scanf("%d",&choice);

>   while(choice!=3)
>   {
>     switch (choice)
>     {
>         case 1:
>           printf("Enter a character: ");
>           scanf("\n%c",&item);
>           insert(&startPtr,item);
>           printList(startPtr);
>           break;

>         case 2:
>           if(!isEmpty(startPtr))
>           {
>             printf("Enter character to be deleted: ");
>             scanf("\n%c",&item);

>             if(delete(&startPtr,item))
>             {
>               printf("%c deleted.\n",item);
>               printList(startPtr);
>             }
>             else
>                 printf("%c not found.\n\n",item);
>           }
>           else
>             printf("List is empty.\n\n");
>             break;
>         default:
>            printf("Invalid choice.\n\n");
>            instructions();
>            break;
>     }
>     printf(">");
>     scanf("%d",&choice);
>   }
>   printf("End of run.\n");
>   return 0;

> }

> void instructions(void)
> {
>    printf("Enter your choice:\n");
>    printf("1 to insert an element into the list.\n"
>           "2 to delete an element from the list.\n"
>           "3 to exit the program.\n");

> }

> void insert(ListNodePtr *sPtr, char value)
> {
>   ListNodePtr newPtr, previousPtr, currentPtr;

>   newPtr=malloc(sizeof(ListNode));
>   if(newPtr !=NULL)
>   {
>     newPtr->data=value;
>     newPtr->nextPtr=NULL;

>    //set the searching index(previousPtr and
>    //currentPtr)to the start of the list.
>     previousPtr=NULL;
>     currentPtr=*sPtr;

>     while(currentPtr !=NULL && value>currentPtr->data)
>     {
>       previousPtr=currentPtr;
>       currentPtr=currentPtr->nextPtr;
>     }  //keep going

>     if(previousPtr==NULL)
>     {
>       newPtr->nextPtr=*sPtr;
>       *sPtr=newPtr;//insert the node at the beginning of the list
>     }
>     else
>     {
>       previousPtr->nextPtr=newPtr;
>       newPtr->nextPtr=currentPtr;
>     }
>   }
>   else
>   {
>     printf("%c not inserted. No memory available.\n",value);
>   }

> }

> char delete(ListNodePtr *sPtr, char value)
> {
>   ListNodePtr previousPtr,currentPtr,tempPtr;

>   if (value==(*sPtr)->data)
>   {
>     tempPtr=*sPtr;
>     *sPtr=(*sPtr)->nextPtr;
>     free(tempPtr);
>     return value;
>   }
>   else
>   {
>     previousPtr=*sPtr;
>     currentPtr=(*sPtr)->nextPtr;//setup the cursor at the beginning

>   //when the cursor didn't reach the tail and cursor didn't find the
>   //specified value, the cursor keep going along the list
>   while(currentPtr !=NULL && currentPtr->data !=value)
>   {
>     previousPtr=currentPtr;
>     currentPtr=currentPtr->nextPtr;//move on to next node
>   }

>   if(currentPtr !=NULL)//if not reaching the tail
>   {
>     tempPtr=currentPtr;
>     previousPtr->nextPtr=currentPtr->nextPtr;
>     free(tempPtr);
>     return value;
>   }
>  }
>  return '\0';//return null char;

> }

> int isEmpty(ListNodePtr sPtr)
> {
>   return sPtr==NULL;

> }

> void printList(ListNodePtr currentPtr)
> {
>   if(currentPtr==NULL)
>   {
>     printf("List is Empty.\n\n");
>   }
>   else
>   {
>     printf("The list is:\n");
>     while(currentPtr !=NULL)
>     {
>         printf("%c--> ",currentPtr->data);
>         currentPtr=currentPtr->nextPtr;
>     }
>     printf("NULL\n\n");
>   }

> }

> ****************************************************************