队列——链表实现

在上一篇文章中，我们介绍了 Queue 并讨论了数组的实现。在这篇文章中，讨论了链表的实现。必须有效地执行以下两个主要操作。
在 Queue 数据结构中，我们维护两个指针， front和back 。前端指向队列的第一项，后端指向最后一项。
enQueue()这个操作在rear之后添加一个新节点并将rear移到下一个节点。
deQueue()此操作移除前端节点，并将前端移动到下一个节点。

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C++

#include 
using namespace std;
 
struct QNode {
    int data;
    QNode* next;
    QNode(int d)
    {
        data = d;
        next = NULL;
    }
};
 
struct Queue {
    QNode *front, *rear;
    Queue()
    {
        front = rear = NULL;
    }
 
    void enQueue(int x)
    {
 
        // Create a new LL node
        QNode* temp = new QNode(x);
 
        // If queue is empty, then
        // new node is front and rear both
        if (rear == NULL) {
            front = rear = temp;
            return;
        }
 
        // Add the new node at
        // the end of queue and change rear
        rear->next = temp;
        rear = temp;
    }
 
    // Function to remove
    // a key from given queue q
    void deQueue()
    {
        // If queue is empty, return NULL.
        if (front == NULL)
            return;
 
        // Store previous front and
        // move front one node ahead
        QNode* temp = front;
        front = front->next;
 
        // If front becomes NULL, then
        // change rear also as NULL
        if (front == NULL)
            rear = NULL;
 
        delete (temp);
    }
};
 
// Driven Program
int main()
{
 
    Queue q;
    q.enQueue(10);
    q.enQueue(20);
    q.deQueue();
    q.deQueue();
    q.enQueue(30);
    q.enQueue(40);
    q.enQueue(50);
    q.deQueue();
    cout << "Queue Front : " << (q.front)->data << endl;
    cout << "Queue Rear : " << (q.rear)->data;
}
// This code is contributed by rathbhupendra

C

// A C program to demonstrate linked list based implementation of queue
#include 
#include 
 
// 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 the
// 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
void deQueue(struct Queue* q)
{
    // If queue is empty, return NULL.
    if (q->front == NULL)
        return;
 
    // 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;
 
    free(temp);
}
 
// Driver Program to test anove 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);
    deQueue(q);
    printf("Queue Front : %d \n", q->front->key);
    printf("Queue Rear : %d", q->rear->key);
    return 0;
}

Java

// Java program for linked-list implementation of queue
 
// A linked list (LL) node to store a queue entry
class QNode {
    int key;
    QNode next;
 
    // constructor to create a new linked list node
    public QNode(int key)
    {
        this.key = key;
        this.next = null;
    }
}
 
// A class to represent a queue
// The queue, front stores the front node of LL and rear stores the
// last node of LL
class Queue {
    QNode front, rear;
 
    public Queue()
    {
        this.front = this.rear = null;
    }
 
    // Method to add an key to the queue.
    void enqueue(int key)
    {
 
        // Create a new LL node
        QNode temp = new QNode(key);
 
        // If queue is empty, then new node is front and rear both
        if (this.rear == null) {
            this.front = this.rear = temp;
            return;
        }
 
        // Add the new node at the end of queue and change rear
        this.rear.next = temp;
        this.rear = temp;
    }
 
    // Method to remove an key from queue.
    void dequeue()
    {
        // If queue is empty, return NULL.
        if (this.front == null)
            return;
 
        // Store previous front and move front one node ahead
        QNode temp = this.front;
        this.front = this.front.next;
 
        // If front becomes NULL, then change rear also as NULL
        if (this.front == null)
            this.rear = null;
    }
}
 
// Driver class
public class Test {
    public static void main(String[] args)
    {
        Queue q = new Queue();
        q.enqueue(10);
        q.enqueue(20);
        q.dequeue();
        q.dequeue();
        q.enqueue(30);
        q.enqueue(40);
        q.enqueue(50);
        q.dequeue();
        System.out.println("Queue Front : " + q.front.key);
        System.out.println("Queue Rear : " + q.rear.key);
    }
}
// This code is contributed by Gaurav Miglani

Python3

# Python3 program to demonstrate linked list
# based implementation of queue
 
# A linked list (LL) node
# to store a queue entry
class Node:
     
    def __init__(self, data):
        self.data = data
        self.next = None
 
# A class to represent a queue
 
# The queue, front stores the front node
# of LL and rear stores the last node of LL
class Queue:
     
    def __init__(self):
        self.front = self.rear = None
 
    def isEmpty(self):
        return self.front == None
     
    # Method to add an item to the queue
    def EnQueue(self, item):
        temp = Node(item)
         
        if self.rear == None:
            self.front = self.rear = temp
            return
        self.rear.next = temp
        self.rear = temp
 
    # Method to remove an item from queue
    def DeQueue(self):
         
        if self.isEmpty():
            return
        temp = self.front
        self.front = temp.next
 
        if(self.front == None):
            self.rear = None
 
# Driver Code
if __name__== '__main__':
    q = Queue()
    q.EnQueue(10)
    q.EnQueue(20)
    q.DeQueue()
    q.DeQueue()
    q.EnQueue(30)
    q.EnQueue(40)
    q.EnQueue(50)
    q.DeQueue()  
    print("Queue Front " + str(q.front.data))
    print("Queue Rear " + str(q.rear.data))

C#

// C# program for linked-list
// implementation of queue
using System;
 
// A linked list (LL) node to
// store a queue entry
class QNode {
    public int key;
    public QNode next;
 
    // constructor to create
    // a new linked list node
    public QNode(int key)
    {
        this.key = key;
        this.next = null;
    }
}
 
// A class to represent a queue The queue,
// front stores the front node of LL and
// rear stores the last node of LL
class Queue {
    QNode front, rear;
 
    public Queue()
    {
        this.front = this.rear = null;
    }
 
    // Method to add an key to the queue.
    public void enqueue(int key)
    {
 
        // Create a new LL node
        QNode temp = new QNode(key);
 
        // If queue is empty, then new
        // node is front and rear both
        if (this.rear == null) {
            this.front = this.rear = temp;
            return;
        }
 
        // Add the new node at the
        // end of queue and change rear
        this.rear.next = temp;
        this.rear = temp;
    }
 
    // Method to remove an key from queue.
    public void dequeue()
    {
        // If queue is empty, return NULL.
        if (this.front == null)
            return;
 
        // Store previous front and
        // move front one node ahead
        QNode temp = this.front;
        this.front = this.front.next;
 
        // If front becomes NULL,
        // then change rear also as NULL
        if (this.front == null)
            this.rear = null;
    }
}
 
// Driver code
public class Test {
    public static void Main(String[] args)
    {
        Queue q = new Queue();
        q.enqueue(10);
        q.enqueue(20);
        q.dequeue();
        q.dequeue();
        q.enqueue(30);
        q.enqueue(40);
        q.enqueue(50);
        q.dequeue();
        Console.WriteLine("Queue Front : " + q.front.key);
        Console.WriteLine("Queue Rear : " + q.rear.key);
    }
}
 
// This code has been contributed by Rajput-Ji

Javascript

// JavaScript program for linked-list implementation of queue
class QNode
{
    constructor(key)
    {
        this.key = key;
        this.next = null;
    }
}
 
let front = null, rear = null;
 
function enqueue(key)
{
    // Create a new LL node
        let temp = new QNode(key);
   
        // If queue is empty, then new node is front and rear both
        if (rear == null) {
            front = rear = temp;
            return;
        }
   
        // Add the new node at the end of queue and change rear
        rear.next = temp;
        rear = temp;
}
 
 
function dequeue()
{
    // If queue is empty, return NULL.
        if (front == null)
            return;
   
        // Store previous front and move front one node ahead
        let temp = front;
        front = front.next;
   
        // If front becomes NULL, then change rear also as NULL
        if (front == null)
            rear = null;
}
 
 
enqueue(10);
enqueue(20);
dequeue();
dequeue();
enqueue(30);
enqueue(40);
enqueue(50);
dequeue();
document.write("Queue Front : " + front.key+"<br>");
document.write("Queue Rear : " + rear.key+"<br>");
 
 
// This code is contributed by avanitrachhadiya2155

输出：

Queue Front : 40
Queue Rear : 50

时间复杂度： enqueue() 和 dequeue() 两个操作的时间复杂度都是 O(1)，因为我们只更改了两个操作中的几个指针。任何操作都没有循环。