class MyStack(object):

def __init__(self, value=None):

self.value = value

# 前驱

self.before = None

# 后继

self.behind = None

def __str__(self):

if self.value is not None:

return str(self.value)

else:

return 'None'

def top(stack):

if isinstance(stack, MyStack):

if stack.behind is not None:

return top(stack.behind)

else:

return stack

def push(stack, ele):

push_ele = MyStack(ele)

if isinstance(stack, MyStack):

stack_top = top(stack)

push_ele.before = stack_top

push_ele.before.behind = push_ele

def pop(stack):

if isinstance(stack, MyStack):

stack_top = top(stack)

if stack_top.before is not None:

stack_top.before.behind = None

stack_top.behind = None

return stack_top

else:

print('已经是栈顶了')