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Monday, July 16, 2007

Introduction to Linked Lists Part II

In the previous article Introduction to Linked Lists, we introduced the basic concept of linked list. To make the program (an the article) as simple as possible, we discussed only the addition and display of nodes in the linked list although necessary we didn't’t discussed the deletion of node in that article.

In this article we will be discussing about the deletion of nodes from linked lists.

Deletion of node (elements) from a linked list

The node to be deleted can be represented by many ways but here we will be representing it by its info. So if we have the following linked list

Linked lists - Deletion of nodes


And we want to delete node1 then we will express it by its info part (i.e. 10).

The main theory behind deletion of nodes is pretty simple. We need to make the link pointer of the node before the target node (to be deleted) to point at the node after the target node. Suppose if we wish to delete node having info as 10 from the above linked list then it will be accomplished as below:

Linked list - Deletion of nodes

Now since node1 is orphan and has no meaning, it can be deleted to free-up memory as represented below:

Linked lists - node deleted to free-up memory

The following example program illustrates all this. Keep reading the comments to understand what is happening where!

  // -- Linked Lists --
  // ------------------
  // Example program in C++
  // to illustrate the most simple
  // linked list
  // NOTE: this program can do three
  // operation [adding, deleting, 
  // and displaying] of data in a 
  // linked list
  #include<iostream.h>

  // node class, this will
  // represent the nodes or elements
  // of the linked list
  class node
  {
   public:
   int info;
   node *link;
  };

  // declare global objects
  node *start=NULL;

  // function prototypes
  void add(int inf);
  void display(void);
  void del(int);

  void main(void)
  {
   int ch;

   // input elements
   while(ch!=0)
   {
    cout<<"enter element to be added:";
    cout<<"\nenter 0 to stop...\n";
    cin>>ch;

    if(ch!=0) add(ch);
    cout<<"\n\n";
   }

   ch=-1;

   while(ch!=0)
   {
    cout<<"enter element to be deleted:";
    cout<<"\nenter 0 to stop...\n";
    cin>>ch;

    if(ch!=0) del(ch);
    cout<<"\n\n";
   }

   cout<<"elements are...\n";
   display();

   // below is a bit confusing
   // part.
   // here all the nodes that
   // we have allocated are
   // being freed up
   node temp;
   while(start!=NULL)
   {
    // store the next node
    // to the one being deleted
    temp=*start;
    // delete the node
    delete start;

    // retrieve the next node
    // to be deleted
    start=temp.link;
   }
  }

  void add(int inf)
  {
   node *temp1;
   node *temp2;

   // if the element to be added
   // is the first element
   if(start==NULL)
   {
    // allocate a new node
    temp1=new node;
    temp1->info=inf;
    temp1->link=NULL;

    // make start point at it
    start=temp1;
   }
   // if not
   else
   {
    temp1=start;

    // find out the last element
    while(temp1->link!=NULL)
    temp1=temp1->link;

    // allocate new node
    temp2=new node;
    temp2->info=inf;
    temp2->link=NULL;

    // make the last element
    // of the list to point
    // at the newly created node
    temp1->link=temp2;
   }
  }

  void display(void)
  {
   node *temp;
   temp=start;

   // traverse or process
   // through each element
   // and keep printing 
   // the information
   while(temp!=NULL)
   {
    cout<<temp->info<<endl;
    temp=temp->link;
   }
  }

  // this function takes an
  // argument which is the info
  // of the node to be delted
  void del(int num)
   {
    node *old;
    node *target;

    target=start;

    while(target!=NULL)
    {
     // if node to be
     // delted is found
     if(target->info==num)
     {
      // if node to be deleted
      // is the first node
      // in the list
      if(target==start)
        start=target->link;

      // if not
      else
        // then make the node
        // prev. to the node
        // to be deleted to 
        // point at the node
        // which is after it
        old->link=target->link;

      // free-up the memory
      delete(target);
      return;
     }
    else
    {
     // traverse through
     // each node
     old=target;
     target=target->link;
    }
   }
  }

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