Explain Linked Lists and queue With example

  Let us now return to our linked list example:

  typedef struct {  int value;
                          ELEMENT *next;
                 } ELEMENT;

We can now try to grow the list dynamically:
  link = (ELEMENT *) malloc(sizeof(ELEMENT));
This will allocate memory for a new link.
If we want to deassign memory from a pointer use the free() function:
  free(link)
See Example programs (queue.c) below and try exercises for further practice.
Full Program: queue.c
A queue is basically a special case of a linked list where one data element joins the list at the left end and leaves in a ordered fashion at the other end.
The full listing for queue.c is as follows:
/*                                 */
/* queue.c                              */
/* Demo of dynamic data structures in C                      */

#include <stdio.h>

#define FALSE 0
#define NULL 0

typedef struct {
    int     dataitem;
    struct listelement *link;
}               listelement;

void Menu (int *choice);
listelement * AddItem (listelement * listpointer, int data);
listelement * RemoveItem (listelement * listpointer);
void PrintQueue (listelement * listpointer);
void ClearQueue (listelement * listpointer);

main () {
    listelement listmember, *listpointer;
    int     data,
            choice;

    listpointer = NULL;
    do {
    Menu (&choice);
    switch (choice) {
        case 1:
        printf ("Enter data item value to add  ");
        scanf ("%d", &data);
        listpointer = AddItem (listpointer, data);
        break;
        case 2:
        if (listpointer == NULL)
            printf ("Queue empty!\n");
        else
            listpointer = RemoveItem (listpointer);
        break;
        case 3:
        PrintQueue (listpointer);
        break;

        case 4:
        break;

        default:
        printf ("Invalid menu choice - try again\n");
        break;
    }
    } while (choice != 4);
    ClearQueue (listpointer);
}                /* main */

void Menu (int *choice) {

    char    local;

    printf ("\nEnter\t1 to add item,\n\t2 to remove item\n\
\t3 to print queue\n\t4 to quit\n");
    do {
    local = getchar ();
    if ((isdigit (local) == FALSE) && (local != '\n')) {
        printf ("\nyou must enter an integer.\n");
        printf ("Enter 1 to add, 2 to remove, 3 to print, 4 to quit\n");
    }
    } while (isdigit ((unsigned char) local) == FALSE);
    *choice = (int) local - '0';
}

listelement * AddItem (listelement * listpointer, int data) {

    listelement * lp = listpointer;

    if (listpointer != NULL) {
    while (listpointer -> link != NULL)
        listpointer = listpointer -> link;
    listpointer -> link = (struct listelement  *) malloc (sizeof (listelement));
    listpointer = listpointer -> link;
    listpointer -> link = NULL;
    listpointer -> dataitem = data;
    return lp;
    }
    else {
    listpointer = (struct listelement  *) malloc (sizeof (listelement));
    listpointer -> link = NULL;
    listpointer -> dataitem = data;
    return listpointer;
    }
}

listelement * RemoveItem (listelement * listpointer) {

    listelement * tempp;
    printf ("Element removed is %d\n", listpointer -> dataitem);
    tempp = listpointer -> link;
    free (listpointer);
    return tempp;
}

void PrintQueue (listelement * listpointer) {

    if (listpointer == NULL)
    printf ("queue is empty!\n");
    else
    while (listpointer != NULL) {
        printf ("%d\t", listpointer -> dataitem);
        listpointer = listpointer -> link;
    }
    printf ("\n");
}

void ClearQueue (listelement * listpointer) {

    while (listpointer != NULL) {
    listpointer = RemoveItem (listpointer);
    }
}