queue.c

// Implementation of Queue in C language using linked list  
#include <stdlib.h> 
#include <stdio.h> 
  
// A linked list (LL) node to store a queue entry 
struct QNode 
{ 
    int key; 
    struct QNode *next; 
}; 
  
// The queue, front stores the front node of LL and rear stores ths 
// last node of LL 
struct Queue 
{ 
    struct QNode *front, *rear; 
}; 
  
// A utility function to create a new linked list node. 
struct QNode* newNode(int k) 
{ 
    struct QNode *temp = (struct QNode*)malloc(sizeof(struct QNode)); 
    temp->key = k; 
    temp->next = NULL; 
    return temp;  
} 
  
// A utility function to create an empty queue 
struct Queue *createQueue() 
{ 
    struct Queue *q = (struct Queue*)malloc(sizeof(struct Queue)); 
    q->front = q->rear = NULL; 
    return q; 
} 
  
// The function to add a key k to q 
void enQueue(struct Queue *q, int k) 
{ 
    // Create a new LL node 
    struct QNode *temp = newNode(k); 
  
    // If queue is empty, then new node is front and rear both 
    if (q->rear == NULL) 
    { 
       q->front = q->rear = temp; 
       return; 
    } 
  
    // Add the new node at the end of queue and change rear 
    q->rear->next = temp; 
    q->rear = temp; 
} 
  
// Function to remove a key from given queue q 
struct QNode *deQueue(struct Queue *q) 
{ 
    // If queue is empty, return NULL. 
    if (q->front == NULL) 
       return NULL; 
  
    // Store previous front and move front one node ahead 
    struct QNode *temp = q->front; 
    q->front = q->front->next; 
  
    // If front becomes NULL, then change rear also as NULL 
    if (q->front == NULL) 
       q->rear = NULL; 
    return temp; 
} 
  
// Driver Program to test above functions 
int main() 
{ 
    struct Queue *q = createQueue(); 
    enQueue(q, 10); 
    enQueue(q, 20); 
    deQueue(q); 
    deQueue(q); 
    enQueue(q, 30); 
    enQueue(q, 40); 
    enQueue(q, 50); 
    struct QNode *n = deQueue(q); 
    if (n != NULL) 
      printf("Dequeued item is %d", n->key); 
    return 0; 
}