TheAlgorithms-C/data_structures/queue.c

////////////////////////////////////////////////////////////////////////////////
// INCLUDES
#include <stdio.h>
#include <stdlib.h>

////////////////////////////////////////////////////////////////////////////////
// MACROS: CONSTANTS

////////////////////////////////////////////////////////////////////////////////
// DATA STRUCTURES
/**
 * Defining the structure of the node which contains 'data' (type : integer), two pointers 'next' and 'pre' (type : struct node).
 */
struct node                                          
{
    int data;
    struct node *next;
    struct node *pre;
} * head, *tail, *tmp;

////////////////////////////////////////////////////////////////////////////////
// GLOBAL VARIABLES
int count;

////////////////////////////////////////////////////////////////////////////////
// FORWARD DECLARATIONS
void create();
void enque(int x);
int deque();
int peek();
int size();
int isEmpty();

////////////////////////////////////////////////////////////////////////////////
// MAIN ENTRY POINT

int main(int argc, char const *argv[])
{
    create();
    enque(5);

    return 0;
}

void create()
{
    head = NULL;
    tail = NULL;
}

/**
 * Puts an item into the Queue.
 */
void enque(int x)
{
    if (head == NULL)
    {
        head = (struct node *)malloc(sizeof(struct node));
        head->data = x;
        head->pre = NULL;
        tail = head;
    }
    else
    {
        tmp = (struct node *)malloc(sizeof(struct node));
        tmp->data = x;
        tmp->next = tail;
        tail = tmp;
    }
}

/**
 * Takes the next item from the Queue.
 */
int deque()
{
    int returnData = 0;
    if (head == NULL)
    {
        printf("ERROR: Deque from empty queue.\n");
        exit(1);
    }
    else
    {
        returnData = head->data;
        if (head->pre == NULL)
            head = NULL;
        else
            head = head->pre;
        head->next = NULL;
    }
    return returnData;
}

/**
 * Returns the size of the Queue.
 */
int size() { return count; }