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"""

Stack based upon linked list

基于链表实现的栈

Author: Wenru

"""

from typing import Optional

class Node:

def __init__(self, data: int, next=None):

self._data = data

self._next = next

class LinkedStack:

"""A stack based upon singly-linked list.

"""

def __init__(self):

self._top: Node = None

def push(self, value: int):

new_top = Node(value)

new_top._next = self._top

self._top = new_top

def pop(self) -> Optional[int]:

if self._top:

value = self._top._data

self._top = self._top._next

return value

def __repr__(self) -> str:

current = self._top

nums = []

while current:

nums.append(current._data)

current = current._next

return " ".join(f"{num}]" for num in nums)

if __name__ == "__main__":

stack = LinkedStack()

for i in range(9):

stack.push(i)

print(stack)

for _ in range(3):

stack.pop()

print(stack)