栈 python

#!/usr/bin/env python3# -*- coding: utf-8 -*-' a test module '__author__ = 'Zhang Shuai'class Node():    def __init__(self,value=None,next=None):        self.next = next        self.value = valueclass stack():    def __init__(self):        self.top = None    def push(self,value):        if self.top is None:            self.top = Node(value)        else:            self.top = Node(value,self.top)    def pop(self):        if self.top:            value = self.top.value            self.top = self.top.next            return value    def peek(self):        if self.top:            return self.top.values = stack()s.push(1)s.push(2)s.push(3)print(s.peek())print(s.pop())print(s.pop())print(s.peek())print(s.pop())print(s.peek())