Queue

Linked Queue

typedef struct N{
    int data;
    struct N* next;
} node;
 
typedef struct{
    node* first;
    node* rear;
} queue;

Enqueue

void enqueue(queue** Q, int num) {
    node* item = (node*)malloc(sizeof(node));
    item->data = num;
    item->next = null;
    if (*Q->rear == null) {
        *Q->first = item;
        *Q->rear = item;
    }
    else {
        *Q->rear->next = item;
        *Q->rear = item;
    }
}

Dequeue

int dequeue(queue** Q) {
    if (*Q->first == null) {
        printf("already empty\n");
    }
    else if (*Q->first == *Q->rear) {
        int ret = *Q->first->data;
        free(*Q->first);
        *Q->first = null;
        *Q->rear = null;
        return ret;
    }
    else {
        int ret = *Q->first-data;
        node* temp = *Q->first;
        *Q->first = *Q->first->next;
        free(*Q->temp);
        return ret;
    }
}

Circular Queue

#define size 100
typedef struct{
    int array[size];
    int first;
    int rear;
    int count;
} queue;

Enqueue

void enqueue(queue** Q, int num) {
    if (*Q->count >= *Q->size) {
        printf("already full\n");
    }
    else {
        *Q->rear = (*Q->rear+1)%(*Q->size);
        *Q->array[*Q->rear] = num;
        *Q->count++;
    }
}

Dequeue

int dequeue(queue** Q) {
    int x = 0;
    if (*Q->count <= 0) {
        printf("already empty\n");
    }
    else {
        x = *Q->array[*Q->first];
        *Q->first = (*Q->first+1)%*(Q->size)
        *Q->count--;
    }
    return x;
}

Other Solutions

Linked Queue

#include <stdio.h>
typedef struct Node{
    struct Node *next;
    int data;
} Node;
 
typedef struct {
    Node *head, *tail;
} Queue;
 
void Enqueue(Queue *Q, int item)
{
    Node *new = (Node*)malloc(sizeof(Node));
    new->data = item;
    new->next = NULL;
 
    if (Q->tail == NULL) {
        Q->head = new;
        Q->tail = new;
    }
    else {
        Q->tail->next = new;
        Q->tail = new;
    }
}
 
Node* Dequeue(Queue *Q)
{
    Node *p;
    if (Q->head == NULL) {
        printf("queue is empty!\n");
        return NULL;
    }
    else if (Q->head == Q->tail) {
        p = Q->head;
        Q->head = Q->tail = NULL;
    }
    else {
        p = Q->head;
        Q->head = Q->head->next;
    }
    return p;
}

Circular Queue

typedef struct {
    int array[100];
    int head, tail, cnt;
} queue;
 
int enqueue(queue *q, int item)
{
    if (q->cnt == 100) {
        printf("queue full!\n");
        return 0;
    }
 
    q->array[q->tail] = item;
    q->tail = (q->tail + 1) % 100;    
    q->cnt += 1;
    return 1;
}
 
int dequeue(queue *q, int *item)
{
    if (q->cnt == 0) {
        printf("queue empty\n");
        return 0;
    }
 
    *item = q->array[head];
    q->head = (q->head + 1) % 100;
    q->cnt -= 1;
    return 1;
}