stack.py

class Node:
    def __init__(self, data, next_node) -> None:
        self.data = data
        self.next_node = next_node


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

    def peek(self):
        return self.top

    def push(self, data):
        next_node = self.top
        new_top = Node(data, next_node)
        self.top = new_top

    def pop(self):
        if self.peek() == None:
            return None
        else:
            removed = self.top
            self.top = removed.next_node
            return removed