如何使用链表实现丢弃堆栈？

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

class DiscardStack:
    def __init__(self, capacity):
        self.top = None
        self.capacity = capacity
        self.size = 0

    def push(self, value):
        new_node = Node(value)
        if self.size == self.capacity:
            self.top = self.top.next
            self.size -= 1
        else:
            self.size += 1
        new_node.next = self.top
        self.top = new_node

    def pop(self):
        if self.top is None:
            return None
        value = self.top.value
        self.top = self.top.next
        self.size -= 1
        return value

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

# 示例使用
stack = DiscardStack(3)
stack.push(1)
stack.push(2)
stack.push(3)
print(stack.pop())  # 输出 3
stack.push(4)
print(stack.peek())  # 输出 4，因为1被丢弃了