Queue

# -*-coding: utf-8 -*-# Author :writen by Qiusheng Li# Time: 2014/08/19# Email:[e-mail]# enqueue Insert an item at the back of the queue# dequeue Remove an item from the front of the queue# peek/front Retrieve an item at front of the queue without removing it# empty/size Check whether the queue is empty or return its size# using list impliment the Queue#node another approach is to implement a queue as a container for nodes,as we have#done for stacks,but now the nodes are inserted and removed in a FIFO orderclass Node(object):    def __init__(self,value=None):        self.value = value        self.next =Noneclass Queue2(object):    '''container nodes to implement a queue class'''    def __init__(self):        self.first =None  # point to the first node        self.pre = None  # point to the last node        def enqueue(self,value):        node  = Node(value)        if self.first == None:            self.first = node        else:          self.pre.next = node        self.pre = node            def dequeue(self):        if self.first:            value = self.first.value  # get value object is better than get node object            self.first =  self.first.next            return value       # else:                         # don't use else, if contains return            raise Exception('Queue is empty')    def peek(self):        if self.first:            return self.first.value        else:            return None            def isEmpty(self):        return bool(self.first)        def size(self):         if self.first == None:             return 0         node = self.first         i = 0         while node:             i += 1             node = node.next         return i     def Queue2Main():    queue = Queue2()    queue.enqueue(1)    queue.enqueue(2)    queue.enqueue(3)    print "size :   ",queue.size()    print "peek:    ",queue.peek()    print "dequeue: ",queue.dequeue()    print "size :   ",queue.size()    print "peek:    ",queue.peek()    print "dequeue: ",queue.dequeue()    print "dequeue: ",queue.dequeue()    print "size :   ",queue.size()                   print "peek:    ",queue.peek()'''testing resultsize :    3peek:     1dequeue:  1size :    2peek:     2dequeue:  2dequeue:  3size :    0peek:     None'''if __name__ == '__main__':    Queue2Main()