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queue.py
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queue.py
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from linked_list import l_list, node
class EmptyQueueError(Exception):
"""An exception that is thrown when trying to dequeue an empty queue"""
def __init__(self, message):
Exception.__init__(self, message)
class _Queue_List(l_list):
def __init__(self):
super(self.__class__, self).__init__()
self.tail = None
def insert_tail(self, val):
if not self.size():
self.insert(val)
else:
self.tail = self.tail.next = node(val, None)
self.num_nodes += 1
def insert(self, val):
super(self.__class__, self).insert(val)
if self.size() == 1:
self.tail = self.head
def remove(self, a_node):
super(self.__class__, self).remove(a_node)
if not self.size():
self.tail = None
def pop(self):
retval = super(self.__class__, self).pop()
if not self.size():
self.tail = None
return retval
class Queue(object):
def __init__(self):
"""initialize the Queue"""
self._ql = _Queue_List()
def enqueue(self, val):
"""add an item to the Queue"""
self._ql.insert_tail(val)
def dequeue(self):
"""remove and return an item from the Queue"""
if not self.size():
raise EmptyQueueError("Queue is empty")
return self._ql.pop()
def size(self):
"""return the number of items in the queue"""
return self._ql.size()