使用节点实现堆栈，顶部节点指针在函数外部是未知的，顶部指针始终为空

class Node:
    def __init__(self, data):
        self.data = data
        self.next = None

class Stack:
    def __init__(self):
        self.top = None

    def push(self, data):
        new_node = Node(data)
        if self.top is None:
            self.top = new_node
        else:
            new_node.next = self.top
            self.top = new_node

    def pop(self):
        if self.top is None:
            return None
        else:
            popped_node = self.top
            self.top = self.top.next
            popped_node.next = None
            return popped_node.data

    def is_empty(self):
        return self.top is None

    def peek(self):
        if self.top is None:
            return None
        else:
            return self.top.data

# 示例用法
stack = Stack()
stack.push(1)
stack.push(2)
stack.push(3)
print(stack.pop())  # 输出：3
print(stack.peek())  # 输出：2
print(stack.is_empty())  # 输出：False