I don't understand the implementation of inserting a new node in linked list

问题

In linked list implementation, the insertion of a new node to the linked list is usually written like this:

void push(struct node** head_ref, int new_data)

/* 1. allocate node */
struct node* new_node = (struct node*) malloc(sizeof(struct node));

/* 2. put in the data  */
new_node->data  = new_data;

/* 3. Make next of new node as head */
new_node->next = (*head_ref);

/* 4. move the head to point to the new node */
(*head_ref)    = new_node;

(I took this code from http://quiz.geeksforgeeks.org/linked-list-set-2-inserting-a-node/)

and the node struct is:

struct node {

int data;

struct node *next;

};

What I don't understand is the 3. and 4. of the insertion part. So you make the next pointer of new_node pointed to the head, and then the head points to the new_node? So that means the next pointer points to new_node?

It seems like a stupid question but I'm having trouble understanding it, so I hope someone can explain it to me. Thank you.

回答1:

Well basically in a linked list all nodes are connected to each other. It depends upon u that where do u insert a new node either at end or start. Each time we insert a new node we will check the head pointer.

   if(head == NULL)  //it means that the node is empty
   {
      head = newNode;  //so we will assign the new node to the head
   }
    else
   {
      node* temp = head;       //creating a temp pointer that will go 
                               // to the end of the linked list

      while(temp -> next != NULL) { temp = temp->next; }
      temp = newNode;          //This will add the new node to the end
      newNode->next = NULL;enter code here 
    }

回答2:

If I understood correctly this is your scenario?

http://www.kkhsou.in/main/EVidya2/computer_science/comscience/313.gif

Your list is just a linked list with next pointer until list's last item that has null as pointer

Step 3 makes your new node to point to 2nd item that was at beginning of the list before this operation

Step 4 makes the list head to point to the new node

Hope this helps

回答3:

/* 1. allocate node / struct node new_node = (struct node*) malloc(sizeof(struct node));

/* 2. put in the data */ new_node->data = new_data;

/* 3. Make next of new node as head */ new_node->next = (*head_ref);

/* 4. move the head to point to the new node */ (*head_ref) = new_node;

In Step1 and 2, a new node is created and data is assigned to it.

When you list is empty, your *head_ref would be null. or else if it has any elements, it would be pointing to that Lets take an example

Now 
*head_ref is null
when input is 1
newnode.data=1
newnode.next=null
*headref=newnode 
now your *headref points to the latest node that is added ,this happens with step4

When you insert 2
newnode.data=2
newnode.next=*headref(to the node which is 1)
newnode=*headref
now your list is
1->2(*headref)
now if you add 3 here
it becomes
1->2->3(*headref)

Hope you understand

回答4:

Rather than explaining it to you, I'm going to suggest a technique that will help you work out the answer for yourself.

Get a piece of paper, a pencil and an eraser.
Draw a box on the paper to represent each variable in your algorithm
Draw the initial linked list:
- Draw a box to represent each existing node in the initial linked list.
- Divide each box into sub-boxes representing the fields.
- In each field write either a value, or a dot representing the "from" end of a pointer.
- For each pointer, draw a line to the thing (e.g. node) that is pointed to, and put an arrowhead on the "to" end. A NULL pointer is a just a dot.
Now execute the algorithm.
- Each time you allocate a new node, draw a new box.
- Each time you assign something to a variable, or a field, rub out the current value and write / draw in the new value or the new dot / arrow.

If you do this carefully and systematically, you will be able to visualize exactly what the list insertion algorithm is doing.

This same technique can be used to visualize any list / tree / graph algorithm ... modulo your ability to get it all onto a sheet of paper, and the paper's ability to survive repeated rub-outs.

^{(This pencil and paper approach is very "old school". As in, this is what we were taught to do when I learned to program in the 1970's. A slightly more modern approach would be to use a whiteboard ...)}

来源：https://stackoverflow.com/questions/40168364/i-dont-understand-the-implementation-of-inserting-a-new-node-in-linked-list

标签

linked-list