C Program For Implementing a Queue Using a Linked List

/*Implementing a Queue Using a Linked List*/

/*which reads an integer and prints its digits in reverse order.

For example, if 12345 is read, the program prints 54321. The digits are extracted, from the right, and stored in a queue.The items in the queue are taken off, one at a time, and printed.*/

#include <stdlib.h>

#include <stdio.h>

typedef struct {

      int num;

} QueueData;

typedef struct node {

      QueueData data;

      struct node *next;

} Node, *NodePtr;

typedef struct queueType {

      NodePtr head, tail;

} QueueType, *Queue;

Queue initQueue() {

      Queue qp = (Queue) malloc(sizeof(QueueType));

      qp -> head = NULL;

      qp -> tail = NULL;

      return qp;

} //end initQueue

int empty(Queue Q) {

      return (Q -> head == NULL);

} //end empty

void enqueue(Queue Q, QueueData d) {

      NodePtr np = (NodePtr) malloc(sizeof(Node));

      np -> data = d;

      np -> next = NULL;

      if (empty(Q)) {

            Q -> head = np;

            Q -> tail = np;

      }

      else {

            Q -> tail -> next = np;

            Q -> tail = np;

      }

} //end enqueue

QueueData dequeue(Queue Q) {

      if (empty(Q)) {

            printf("\nAttempt to remove from an empty queue\n");

            exit(1);

      }

      QueueData hold = Q -> head -> data;

      NodePtr temp = Q -> head;

      Q -> head = Q -> head -> next;

      if (Q -> head == NULL) Q -> tail = NULL;

      free(temp);

      return hold;

} //end dequeue

main() {

      int n;

      QueueData temp;

      Queue Q = initQueue();

      printf("Enter a positive integer: ");

      scanf("%d", &n);

      while (n > 0) {

            temp.num = n % 10;

            enqueue(Q, temp);

            n = n / 10;

      }

      printf("\nDigits in reverse order: ");

      while (!empty(Q))

            printf("%d", dequeue(Q).num);

      printf("\n");

} //end main