class Stack:

     def __init__(self):

         self._first = None

     def push(self,item):

         self._first = _Node(item,self._first)

     def pop(self):

         self._first = self._first.next

     def isEmpty(self):

         return self._first is None

 class _Node:

     def __init__(self,item,next):

         self.item = item

         self.next = next

 def main():

     stack = Stack()

     stack.push("a")

     stack.push("b")

     stack.push("c")

     print(stack._first.item)

     print(stack._first.next.item)

     print(stack._first.next.next.item)

 if __name__ == "__main__": main() 
运行结果：
>> c
>> b
>> a