Implementation of linked list deletion operation:

Linked List:

A linear data structure where each element denotes a separate object is known as linked list.

• Each element of a list contains two items, the data and a pointer to next node.
• The last node would point to null.
• The "head" denotes point of entry into a linked list.
• If list is empty then the head is a null pointer.
• The structure of linked list is given below:

EBK DATA STRUCTURES AND ALGORITHMS IN C, Chapter 3, Problem 7E

/*************************************************************

* This program demonstrates deletion of linked list     *

* nodes corresponding to positions obtained from        *

* two linked  lists                                         *

*************************************************************/

Program:

//Include header files

#include<iostream>

using namespace std;

//Declare an array "la[]"

int la[100], i=0;

//Define a linked list node

struct lnode

{

    //Define data of node

    int ldata;

    //Define pointer to next node

    lnode *lnext;

}*start;

/*Function Prototypes */

lnode *lcreate_node(int lvalue);

void lsortedInsert(struct lnode** head_ref, struct lnode* lnew_node);

void ldisplay(struct lnode* head);

void ldeleteKey(struct lnode **head_ref, int lkey);

void merge(struct lnode *p, struct lnode **q);

struct lnode* lSortedMerge(struct lnode* la, struct lnode* lb);

void lFBS(struct lnode* lsource, struct lnode** frontRef, struct lnode** backRef);

/* Function "lcreate_node()" creates la new node, allocates the memory space and puts the data in it */

struct lnode *lcreate_node(int lnew_data)

{

    //Allocate lnode

    struct lnode* lnew_node = (struct lnode*)     malloc(sizeof(struct lnode));

    // Put in the ldata

    lnew_node->ldata = lnew_data;

    //Make the next of new node to NULL

    lnew_node->lnext = NULL;

    //Return the new node

    return lnew_node;

}

//Define "merge()" that merges two linked lists

void merge(struct lnode *p, struct lnode **q)

{

    //Declare nodes of type "lnode*"

    struct lnode *lp_curr = p, *lq_curr = *q;

    struct lnode *lp_next, *lq_next;

    // Loop until positions are available

    while (lp_curr != NULL && lq_curr != NULL)

    {

        // Save the next pointers

        lp_next = lp_curr->lnext;

        lq_next = lq_curr->lnext;

        // Make lq_curr as next of lp_curr

        lq_curr->lnext = lp_next;

        lp_curr->lnext = lq_curr;

        // Update the pointers

        lp_curr = lp_next;

        lq_curr = lq_next;

}

        // Update second list's head pointer

        *q = lq_curr;

}

//Define a function "MergeSort()" that sorts linked list by changing next pointers

void MergeSort(struct lnode** headRef)

{

    //Declare nodes of type "lnode*"

    struct lnode* head = *headRef;

    struct lnode* la;

    struct lnode* lb;

    //If linked list is empty or has single element

    if ((head == NULL) || (head->lnext == NULL))

    {

        return;

    }

    // Split head into sublists

    lFBS(head, &la, &lb);

    // Sort sublists

    MergeSort(&la);

    MergeSort(&lb);

    // Merge lists that are sorted

    *headRef = lSortedMerge(la, lb);

}

/*Define a function "lFBS()" that divides the list into two halves and returns refernce parameters of result*/

void lFBS(struct lnode* lsource, struct lnode** frontRef, struct lnode** backRef)

{

    //Declare nodes "fast" and "slow" of type "lnode*"

    struct lnode* fast;

    struct lnode* slow;

    //If list is empty or contains single element

    if (lsource==NULL || lsource->lnext==NULL)

    {

        //If length < 2

        *frontRef = lsource;

        *backRef = NULL;

    }

    else

    {

        //If list contains more than one element

        slow = lsource;

        fast = lsource->lnext;

        /* Traverse "fast" two nodes, and traverse "slow"         one node */

        while (fast != NULL)

        {

            //Traverse the "fast"

            fast = fast->lnext;

            /* Traverse the list until "fast" reaches                 null */

            if (fast != NULL)

            {

                //Move to next element

                slow = slow->lnext;

                fast = fast->lnext;

            }

        }

        /* "slow" is before list's midpoint, split it in             two at that point. */

        *frontRef = lsource;

        *backRef = slow->lnext;

        slow->lnext = NULL;

    }

}

/* Define a function "lSortedMerge()" that merges the lists that are sorted, it takes header pointers of two lists as arguments */

struct lnode* lSortedMerge(struct lnode* la, struct lnode* lb)

{

    //Declare node to store result

    struct lnode* result = NULL;

    //If either of list is empty

    if (la == NULL)

    return(lb);

    else if (lb==NULL)

    return(la);

    //Chose either la or lb, and recur

    if (la->ldata <= lb->ldata)

    {

        //Place "la" first

         result = la;

        //Continue with next element of "la" and "lb"

        result->lnext = lSortedMerge(la->lnext, lb);

    }

    else

    {

        //place "lb" first

        result = lb;

        //Continue with next element of "lb" and "la"

        result->lnext = lSortedMerge(la, lb->lnext);

    }

    //Return result

    return(result);

}

//Define a main function

int main()

{

    //Initialize variables

    start = NULL;

    //Make the head reference of new node to NULL

    struct lnode* head = NULL;

    //Insert first node into linked list

    struct lnode *lnew_node = lcreate_node(5);

    /*Call the function "lsortedInsert()" with "head" and     "lnew_node" as arguments and inserts the node into    it.*/

    lsortedInsert(&head, lnew_node);

    lnew_node = lcreate_node(10);

    lsortedInsert(&head, lnew_node);

    lnew_node = lcreate_node(20);

    lsortedInsert(&head, lnew_node);

    lnew_node = lcreate_node(30);

    lsortedInsert(&head, lnew_node);

    lnew_node = lcreate_node(40);

    lsortedInsert(&head, lnew_node);

    lnew_node = lcreate_node(50);

    lsortedInsert(&head, lnew_node);

    lnew_node = lcreate_node(60);

    lsortedInsert(&head, lnew_node);

    lnew_node = lcreate_node(70);

    lsortedInsert(&head, lnew_node);

    lnew_node = lcreate_node(80);

    //Display the elements of list

    cout<<"Elements of first list are: "<<endl;

    //Call the function "ldisplay()" to display elements.

    ldisplay(head);

    //Make the head reference of new node to NULL

    struct lnode* head1 = NULL;

    //Insert the first node into linked list

    struct lnode *new_node1 = lcreate_node(3);

    lsortedInsert(&head1, new_node1);

    /*Call the function "lsortedInsert()" with "head1"     and "new_node1" as arguments and inserts the node into     it.*/

    new_node1 = lcreate_node(1);

    lsortedInsert(&head1, new_node1);

    new_node1 = lcreate_node(2);

    lsortedInsert(&head1, new_node1);

    //Display elements of linked list

    cout<<"Elements of second list are: "<<endl;

    ldisplay(head1);

    //Make the head reference of new node to NULL

    struct lnode* head2 = NULL;

    //Insert the first node into linked list

    struct lnode *new_node2 = lcreate_node(6);

    lsortedInsert(&head2, new_node2);

    /*Call the function "lsortedInsert()" with "head1"     and "new_node1" as arguments and inserts the node into     it.*/

    new_node2 = lcreate_node(5);

    lsortedInsert(&head2, new_node2);

    new_node2 = lcreate_node(4);

    lsortedInsert(&head2, new_node2);

    //Display elements of linked list

    cout<<"Elements of third list are: "<<endl;

    ldisplay(head2);

    //Call "merge()" to merge two linked lists

     merge( head1, &head2);

    printf("Linked List after merging second and third     list: \n");

    MergeSort(&head1);

    ldisplay(head1);

    //Store the values of second list into an array

    while( head1!=NULL )

    {

        //Copy the data at the node into variable "c"

        int c = head1->ldata;

        //copy value in "c" into the array "la[]"

        la[i] = c;

        //Increment value of "i"

        i++;

        // Traverse each element

        head1 = head1->lnext;

    }

    //Declare the variables

    int lIndex = 0;

    /*Search the element in first list with the position     in second list */

    for(int k=0;k<i;k++)

    {

        /*Declare la node "ltemp" and assign "head "into             it */

        struct lnode* ltemp = head;

        //Declare the variables

        lIndex=0;

        /*Match the values in first array with index in             second array */

        while(lIndex !=la[k] && ltemp!=NULL)

        {

            //Traverse the list

            ltemp = ltemp->lnext;

            //Increment the index value

            lIndex++;

        }

        /*Assign the value 0 to data that are at                 positions in second list */

        ltemp->ldata = 0;

    }

    /*Call the function "ldeleteKey()" with "head" and     value 0 as arguments */

    ldeleteKey(&head,0);

    //Display the result after deletion

    cout<<"Elements of first list after deletion :     "<<endl;

    //Call "ldisplay()" to display list

    ldisplay(head);

    cout<<endl;

    //Pause the console window

    system("pause");

    return 0;

}

/*Declare the function "lsortedInsert()" that takes head reference and new node as arguments and inserts the node into list in sorted order */

void lsortedInsert(struct lnode** head_ref, struct lnode* lnew_node)

{

    //Declare la node "current"

    struct lnode* current;

    /*If list is empty or data of new node is less than     or equal to present data of list */

    if (*head_ref == NULL || (*head_ref)->ldata >=     lnew_node->ldata)

    {

        //Make head reference to "lnew_node->lnext"

        lnew_node->lnext = *head_ref;

        //Assign new node to head reference

        *head_ref = lnew_node;

    }

    else

    {

        //Locate node before insertion point

        current = *head_ref;

        //Place the node in sorted order

        while (current->lnext!=NULL && current->lnext->

  ldata < lnew_node->ldata)

        {

            //Traverse the list pointer

            current = current->lnext;

        }

        //Make "current->lnext" to "lnew_node->lnext"

        lnew_node->lnext = current->lnext;

        //Assign new node to "current->lnext"

        current->lnext = lnew_node;

    }

}

/*The function "ldisplay()" takes the header pointer of list as arguments and elements of list are displayed */

void ldisplay(struct lnode* head)

{

    //Declare a node "ltemp"

    struct lnode *ltemp;

    //If head is NULL

    if (head == NULL)

    {

        //Display the message

        cout<<"The List is Empty"<<endl;

        return;

    }

    //Set "ltemp" as head node

    ltemp = head;

    /*Display the data in linked list until it reaches     null */

    while (ltemp != NULL)

    {

        //Display the data

        cout<<ltemp->ldata<<"->";

        //Move to next element

        ltemp = ltemp->lnext;

    }

    //Display the end of list

    cout<<"NULL"<<endl;

}

/*The function ldeletekey()" takes the header pointer and the deletion element as arguments and deletes the particular element from the list */

void ldeleteKey(struct lnode **head_ref, int lkey)

{

    // Store head node

    struct lnode* ltemp = *head_ref, *prev;

    /*Check for all occurrences of the deletion element in     the list */

    while (ltemp != NULL && ltemp->ldata == lkey)

    {

        //Change header pointer

        *head_ref = ltemp->lnext;

        //Free old head

        free(ltemp);

        //change "ltemp"

        ltemp = *head_ref;

    }

    //Delete all occurrences of element other than "head"

    while (ltemp != NULL)

    {

        /*Search for key to be deleted, track previous             node as 'prev->lnext' is to be changed*/

        while (ltemp != NULL && ltemp->ldata != lkey)

        {

            prev = ltemp;

            ltemp = ltemp->lnext;

        }

        //If key is not in list

        if (ltemp == NULL) return;

        //Unlink the node from list

        prev->lnext = ltemp->lnext;

        //Free memory

        free(ltemp);

        //Update "ltemp" for next loop iteration

        ltemp = prev->lnext;

    }

}

Explanation:

• The above program declares two linked list and deletes elements of first linked list at positions corresponding to elements in second linked list.
• The function “lcreate_node()” takes an integer value as parameter and creates a node of the linked list.
• The function “lsortedInsert()” takes header pointers of list and new node created as the parameters of the function and inserts the node in the list in sorted order.
• The function “ldisplay()” takes header pointers of linked list as arguments and displays the linked list contents.
• The function “ldeleteKey()” takes header reference pointer of linked list and element to delete as the function arguments and it deletes all element occurrences from list.
• The function “merge()” takes header reference pointer of two linked list as arguments and merges two lists.
• The function “lSortedMerge()”that merges the list that are sorted, it takes header pointers of two list as arguments.
• The function “lFBS()”that divides the list into two halves and returns reference parameters of result.
• In the main function three linked list are defined, elements of second list and third list are merged and then sorted which denotes deletion position index of first list.
• The data values in first list corresponding to positions in merged list are replaced with value 0.
• Delete function is called with header pointer of list and value 0 as function arguments so as to delete all occurrences of 0 from list.
• The final linked after the deletion process is displayed as result.

Output:

Elements of first list are:

5->10->20->30->40->50->60->70->NULL

Elements of second list are:

1->2->3->NULL

Elements of third list are:

4->5->6->NULL

Linked List after merging second and third list:

1->2->3->4->5->6->NULL

Elements of first list after deletion:

5->70->NULL

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