[Python数据结构] 使用 Circular List实现Queue

[Python数据结构] 使用 Circular List实现Queue

1. Queue
队列，又称为伫列（queue），是先进先出（FIFO, First-In-First-Out）的线性表。在具体应用中通常用链表或者数组来实现。队列只允许在后端（称为rear）进行插入操作，在前端进行删除操作。队列的操作方式和堆栈类似，唯一的区别在于队列只允许新数据在后端进行添加。

Queue[维基百科]

2. Queue ADT
队列是一种抽象数据类型，其实例Q需要支持两种方法：
　　1）Q.enqueue(e)  : Add an element to the back of queue.
　　2）Q.dequeue( )  : Remove and return the first element in the queue

另外，为了方便使用，我们还定义了以下方法：
　　3）Q.first() : Return the element at the front of the queue
　　4）Q.is_empty() : Return True if the queue is empty
　　5）len(Q) : Return the number of elements in the queue

3. Queue Implementation

class Empty(Exception):
    """Error attempting to access an element from an empty container"""
    pass

class ArrayQueue():
    """FIFO queue implementation using a python list as underlying storage."""
    Default_capacity = 10
    
    def __init__(self):
        """Create an empty queue"""
        self.data = [None] * ArrayQueue.Default_capacity  # reference to a list instance with a fixed capacity.
        self.size = 0  # an integer representing the current number of elements stored in the queue.
        self.front = 0  # an integer that represents the index within data of the first element of the queue.
        
    def __len__(self):
        """Return the number od elements in the queue."""
        return self.size
    
    def is_empty(self):
        """Return True if the queue is empty."""
        return self.size == 0
    
    def first(self):
        """Return the element at the front of the queue.
        
        Raise Empty exception if the queue is empty."""
        if self.is_empty():
            raise Empty('the queue is empty')
        return self.data[self.front]
           
    def dequeue(self):
        """Remove and return the first element in the queue.
        
        Raise Empty exception if the queue is empty."""
        if self.is_empty():
            raise Empty('the queue is empty')
        answer = self.data[self.front]
        self.data[self.front] = None
        self.front = (self.front + 1) % len(self.data)
        self.size -= 1
        return answer
        
    def enqueue(self, e):
        """Add an element to the back of queue."""
        if self.size == len(self.data):
            self.resize(2 * len(self.data))
        avail = (self.front + self.size) % len(self.data)
        self.data[avail] = e
        self.size += 1  
        
    def resize(self, cap):
        """Resize to a new list of capacity."""
        old = self.data
        self.data = [None] * cap
        walk = self.front
        for k in range(self.size):
            self.data[k] = old[walk]
            walk = (1 + walk) % len(old)
        self.front = 0

4. 执行结果：

q = ArrayQueue()
l = np.random.randint(0, 10, size=(20, ))
for i in l:
    q.enqueue(i)

q.data
[0, 1, 5, 3, 9, 5, 0, 9, 1, 0, 4, 6, 7, 0, 2, 5, 9, 3, 3, 9]

q.dequeue()
0

q.data
[None, 1, 5, 3, 9, 5, 0, 9, 1, 0, 4, 6, 7, 0, 2, 5, 9, 3, 3, 9]

q.first()
1

q.data
[None, 1, 5, 3, 9, 5, 0, 9, 1, 0, 4, 6, 7, 0, 2, 5, 9, 3, 3, 9]