[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]