stack_ll.py

from typing import Optional, Any


class StackUnderFlowError(Exception):
    def __init__(self):
        super().__init__('Stack Under Flow error')


class Node:
    def __init__(self, data: Any = None) -> None:
        self.value: Any = data 
        self.next: Optional[Node] = None


class StackLL:

    def __init__(self) -> None:
        self.top: Optional[Node] = None
        self.size: int = 0


    def push(self, element: Any) -> None:
        new_node: Node = Node(element)
        new_node.next = self.top # None
        self.top = new_node


    def pop(self) -> Any:
        if self.empty:
            raise StackUnderFlowError()
        
        element: Any = self.top.value
        self.top = self.top.next
        return element


    def peek(self) -> Any:
        if self.empty:
            raise StackUnderFlowError()
        
        return self.top.value


    @property
    def empty(self) -> bool:
        return self.top is None