用Python实现栈---2链表

class Node:
    def __init__(self, data, _next = None):
        self.data = data #数据域
        self.next = _next #指针域

class Stack:
    def __init__(self):
        #以链表的第一个节点作为栈顶
        self.__top = None
        self._size = 0

    def push(self, item):
        #添加一个元素item到栈顶
        #让self.top指向新的节点
        #让新的节点得next指向原本的栈顶
        self.__top = Node(item, self.__top)
        self._size += 1

    def pop(self):
        if self.is__empty():
            raise ValueError("栈为空")
        value =  self.__top.data
        self.__top = self.__top.next
        self._size -= 1
        return value

    def top(self):  # 返回栈顶元素
        if self.is__empty():
            raise ValueError("栈为空")
        return self.__top.data

    def is__empty(self):
        return self._size == 0

    def size(self):
        return len(self._size)

if __name__ == '__main__':
    stack = Stack()
    stack.push(1)
    stack.push(2)
    stack.push(3)
    stack.push(4)
    print(stack.pop())
    print(stack.pop())
    print(stack.pop())
    print(stack.pop())
    print(stack.pop())