"""
 链式队列
"""


# 队列异常
class QueueError(Exception):
 pass


class Node:
 def __init__(self, val, next=None):
 self.val = val
 self.next = next


class LinkQueue:
 def __init__(self):
 self.front = self.rear = Node(None)

 def is_empty(self):
 return self.front is self.rear

 def enqueue(self, elem):
 """
 入队
 :param elem:
 :return:
 """
 self.rear.next = Node(elem)
 self.rear = self.rear.next

 def dequeue(self):
 """
 出队
 :return:
 """
 if self.front == self.rear:
 raise QueueError('Queue is empty')

 self.front = self.front.next
 return self.front.val

 def clear(self):
 """
 清空队列
 :return:
 """
 self.front = self.rear


if __name__ == "__main__":
 lq = LinkQueue()
 print(lq.is_empty())
 lq.enqueue(10)
 lq.enqueue(20)
 lq.enqueue(30)
 while not lq.is_empty():
 print(lq.dequeue())