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📜  队列 | Set 1(介绍和数组实现)

📅  最后修改于: 2022-05-13 01:57:00.763000             🧑  作者: Mango

队列 | Set 1（介绍和数组实现）

与堆栈一样，队列是一个线性结构，它遵循执行操作的特定顺序。顺序是先入先出 ( FIFO )。队列的一个很好的例子是资源的任何消费者队列，其中先来的消费者首先得到服务。
堆栈和队列之间的区别在于删除。在堆栈中，我们删除最近添加的项目；在队列中，我们删除最近最少添加的项目。
队列操作：
主要对队列进行以下四个基本操作：
入队：将项目添加到队列中。如果队列已满，则称其为溢出条件。
Dequeue：从队列中移除一个项目。项目以与推送相同的顺序弹出。如果队列为空，则称其为下溢条件。
Front：从队列中获取最前面的项目。
后：从队列中获取最后一项。

队列

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队列的应用：
当不需要立即处理事物，但必须像广度优先搜索一样按优先顺序处理时使用队列。 Queue 的这个属性使其在以下场景中也很有用。
1)当一个资源在多个消费者之间共享时。示例包括 CPU 调度、磁盘调度。
2)当数据在两个进程之间异步传输时（数据的接收速率不一定与发送速率相同）。示例包括 IO 缓冲区、管道、文件 IO 等。
有关 Queue 和 Stack 的更多详细应用，请参见此处。
Queue 的数组实现
为了实现队列，我们需要跟踪两个索引，前端和后端。我们在后部入队并从前部出队。如果我们简单地增加前后索引，那么可能会出现问题，前面可能会到达数组的末尾。解决这个问题的办法是增加前后圆形。



C++
// CPP program for array
// implementation of queue
#include 
using namespace std;
 
// A structure to represent a queue
class Queue {
public:
    int front, rear, size;
    unsigned capacity;
    int* array;
};
 
// function to create a queue
// of given capacity.
// It initializes size of queue as 0
Queue* createQueue(unsigned capacity)
{
    Queue* queue = new Queue();
    queue->capacity = capacity;
    queue->front = queue->size = 0;
 
    // This is important, see the enqueue
    queue->rear = capacity - 1;
    queue->array = new int[queue->capacity];
    return queue;
}
 
// Queue is full when size
// becomes equal to the capacity
int isFull(Queue* queue)
{
    return (queue->size == queue->capacity);
}
 
// Queue is empty when size is 0
int isEmpty(Queue* queue)
{
    return (queue->size == 0);
}
 
// Function to add an item to the queue.
// It changes rear and size
void enqueue(Queue* queue, int item)
{
    if (isFull(queue))
        return;
    queue->rear = (queue->rear + 1)
                  % queue->capacity;
    queue->array[queue->rear] = item;
    queue->size = queue->size + 1;
    cout << item << " enqueued to queue\n";
}
 
// Function to remove an item from queue.
// It changes front and size
int dequeue(Queue* queue)
{
    if (isEmpty(queue))
        return INT_MIN;
    int item = queue->array[queue->front];
    queue->front = (queue->front + 1)
                   % queue->capacity;
    queue->size = queue->size - 1;
    return item;
}
 
// Function to get front of queue
int front(Queue* queue)
{
    if (isEmpty(queue))
        return INT_MIN;
    return queue->array[queue->front];
}
 
// Function to get rear of queue
int rear(Queue* queue)
{
    if (isEmpty(queue))
        return INT_MIN;
    return queue->array[queue->rear];
}
 
// Driver code
int main()
{
    Queue* queue = createQueue(1000);
 
    enqueue(queue, 10);
    enqueue(queue, 20);
    enqueue(queue, 30);
    enqueue(queue, 40);
 
    cout << dequeue(queue)
         << " dequeued from queue\n";
 
    cout << "Front item is "
         << front(queue) << endl;
    cout << "Rear item is "
         << rear(queue) << endl;
 
    return 0;
}
 
// This code is contributed by rathbhupendra

C
// C program for array implementation of queue
#include 
#include 
#include 
 
// A structure to represent a queue
struct Queue {
    int front, rear, size;
    unsigned capacity;
    int* array;
};
 
// function to create a queue
// of given capacity.
// It initializes size of queue as 0
struct Queue* createQueue(unsigned capacity)
{
    struct Queue* queue = (struct Queue*)malloc(
        sizeof(struct Queue));
    queue->capacity = capacity;
    queue->front = queue->size = 0;
 
    // This is important, see the enqueue
    queue->rear = capacity - 1;
    queue->array = (int*)malloc(
        queue->capacity * sizeof(int));
    return queue;
}
 
// Queue is full when size becomes
// equal to the capacity
int isFull(struct Queue* queue)
{
    return (queue->size == queue->capacity);
}
 
// Queue is empty when size is 0
int isEmpty(struct Queue* queue)
{
    return (queue->size == 0);
}
 
// Function to add an item to the queue.
// It changes rear and size
void enqueue(struct Queue* queue, int item)
{
    if (isFull(queue))
        return;
    queue->rear = (queue->rear + 1)
                  % queue->capacity;
    queue->array[queue->rear] = item;
    queue->size = queue->size + 1;
    printf("%d enqueued to queue\n", item);
}
 
// Function to remove an item from queue.
// It changes front and size
int dequeue(struct Queue* queue)
{
    if (isEmpty(queue))
        return INT_MIN;
    int item = queue->array[queue->front];
    queue->front = (queue->front + 1)
                   % queue->capacity;
    queue->size = queue->size - 1;
    return item;
}
 
// Function to get front of queue
int front(struct Queue* queue)
{
    if (isEmpty(queue))
        return INT_MIN;
    return queue->array[queue->front];
}
 
// Function to get rear of queue
int rear(struct Queue* queue)
{
    if (isEmpty(queue))
        return INT_MIN;
    return queue->array[queue->rear];
}
 
// Driver program to test above functions./
int main()
{
    struct Queue* queue = createQueue(1000);
 
    enqueue(queue, 10);
    enqueue(queue, 20);
    enqueue(queue, 30);
    enqueue(queue, 40);
 
    printf("%d dequeued from queue\n\n",
           dequeue(queue));
 
    printf("Front item is %d\n", front(queue));
    printf("Rear item is %d\n", rear(queue));
 
    return 0;
}

Java
// Java program for array
// implementation of queue
 
// A class to represent a queue
class Queue {
    int front, rear, size;
    int capacity;
    int array[];
 
    public Queue(int capacity)
    {
        this.capacity = capacity;
        front = this.size = 0;
        rear = capacity - 1;
        array = new int[this.capacity];
    }
 
    // Queue is full when size becomes
    // equal to the capacity
    boolean isFull(Queue queue)
    {
        return (queue.size == queue.capacity);
    }
 
    // Queue is empty when size is 0
    boolean isEmpty(Queue queue)
    {
        return (queue.size == 0);
    }
 
    // Method to add an item to the queue.
    // It changes rear and size
    void enqueue(int item)
    {
        if (isFull(this))
            return;
        this.rear = (this.rear + 1)
                    % this.capacity;
        this.array[this.rear] = item;
        this.size = this.size + 1;
        System.out.println(item
                           + " enqueued to queue");
    }
 
    // Method to remove an item from queue.
    // It changes front and size
    int dequeue()
    {
        if (isEmpty(this))
            return Integer.MIN_VALUE;
 
        int item = this.array[this.front];
        this.front = (this.front + 1)
                     % this.capacity;
        this.size = this.size - 1;
        return item;
    }
 
    // Method to get front of queue
    int front()
    {
        if (isEmpty(this))
            return Integer.MIN_VALUE;
 
        return this.array[this.front];
    }
 
    // Method to get rear of queue
    int rear()
    {
        if (isEmpty(this))
            return Integer.MIN_VALUE;
 
        return this.array[this.rear];
    }
}
 
// Driver class
public class Test {
    public static void main(String[] args)
    {
        Queue queue = new Queue(1000);
 
        queue.enqueue(10);
        queue.enqueue(20);
        queue.enqueue(30);
        queue.enqueue(40);
 
        System.out.println(queue.dequeue()
                           + " dequeued from queue\n");
 
        System.out.println("Front item is "
                           + queue.front());
 
        System.out.println("Rear item is "
                           + queue.rear());
    }
}
 
// This code is contributed by Gaurav Miglani

Python3
# Python3 program for array implementation of queue
 
# Class Queue to represent a queue
class Queue:
 
    # __init__ function
    def __init__(self, capacity):
        self.front = self.size = 0
        self.rear = capacity -1
        self.Q = [None]*capacity
        self.capacity = capacity
     
    # Queue is full when size becomes
    # equal to the capacity
    def isFull(self):
        return self.size == self.capacity
     
    # Queue is empty when size is 0
    def isEmpty(self):
        return self.size == 0
 
    # Function to add an item to the queue.
    # It changes rear and size
    def EnQueue(self, item):
        if self.isFull():
            print("Full")
            return
        self.rear = (self.rear + 1) % (self.capacity)
        self.Q[self.rear] = item
        self.size = self.size + 1
        print("% s enqueued to queue"  % str(item))
 
    # Function to remove an item from queue.
    # It changes front and size
    def DeQueue(self):
        if self.isEmpty():
            print("Empty")
            return
         
        print("% s dequeued from queue" % str(self.Q[self.front]))
        self.front = (self.front + 1) % (self.capacity)
        self.size = self.size -1
         
    # Function to get front of queue
    def que_front(self):
        if self.isEmpty():
            print("Queue is empty")
 
        print("Front item is", self.Q[self.front])
         
    # Function to get rear of queue
    def que_rear(self):
        if self.isEmpty():
            print("Queue is empty")
        print("Rear item is",  self.Q[self.rear])
 
 
# Driver Code
if __name__ == '__main__':
 
    queue = Queue(30)
    queue.EnQueue(10)
    queue.EnQueue(20)
    queue.EnQueue(30)
    queue.EnQueue(40)
    queue.DeQueue()
    queue.que_front()
    queue.que_rear()

C#
// C# program for array implementation of queue
using System;
 
namespace GeeksForGeeks {
// A class to represent a linearqueue
class Queue {
    private int[] ele;
    private int front;
    private int rear;
    private int max;
 
    public Queue(int size)
    {
        ele = new int[size];
        front = 0;
        rear = -1;
        max = size;
    }
 
    // Function to add an item to the queue.
    // It changes rear and size
    public void enqueue(int item)
    {
        if (rear == max - 1) {
            Console.WriteLine("Queue Overflow");
            return;
        }
        else {
            ele[++rear] = item;
        }
    }
 
    // Function to remove an item from queue.
    // It changes front and size
    public int dequeue()
    {
        if (front == rear + 1) {
            Console.WriteLine("Queue is Empty");
            return -1;
        }
        else {
            Console.WriteLine(ele[front] + " dequeued from queue");
            int p = ele[front++];
            Console.WriteLine();
            Console.WriteLine("Front item is {0}", ele[front]);
            Console.WriteLine("Rear item is {0} ", ele[rear]);
            return p;
        }
    }
 
    // Function to print queue.
    public void printQueue()
    {
        if (front == rear + 1) {
            Console.WriteLine("Queue is Empty");
            return;
        }
        else {
            for (int i = front; i <= rear; i++) {
                Console.WriteLine(ele[i] + " enqueued to queue");
            }
        }
    }
}
 
// Driver code
class Program {
    static void Main()
    {
        Queue Q = new Queue(5);
 
        Q.enqueue(10);
        Q.enqueue(20);
        Q.enqueue(30);
        Q.enqueue(40);
        Q.printQueue();
        Q.dequeue();
    }
}
}

输出： 

10 enqueued to queue
20 enqueued to queue
30 enqueued to queue
40 enqueued to queue
10 dequeued from queue
Front item is 20
Rear item is 40

复杂度分析：

  • 时间复杂度： 
Operations              Complexity
Enque(insertion)           O(1)
Deque(deletion)            O(1)
Front(Get front)           O(1)
Rear(Get Rear)             O(1)
  • 辅助空间： O(N)。
    N 是用于存储元素的数组的大小。

数组实现的优点：

  1. 易于实施。

数组实现的缺点：

  1. 静态数据结构，固定大小。
  2. 如果队列有大量的入队和出队操作，在某些时候（在前后索引线性递增的情况下），即使队列为空，我们也可能无法在队列中插入元素（避免了这个问题）通过使用循环队列）。