class Node(object):
def __init__(self,value):
self.value=value
self.next=None
class stack(object):
def __init__(self):
self.top=None
def peek(self):
if self.top!=None:
return self.top.value
else:
return None
def push(self,value):
node=Node(value)
node.next=self.top
self.top=node
return node.value
def pop(self):
if self.top==None:
return None
else:
tmp=self.top.value
self.top=self.top.next
return tmp
if __name__ =='__main__':
s=stack()
s.push(1)
s.push(2)
s.push(3)
print(s.pop())
print(s.pop())
print(s.pop())