""" A skeleton of Stack using singly linked list.
    Students need to complete the implementation.
    CSCI 204, Spring 2020
    Xiannong Meng
"""
class Node:
    '''
    A simple Node class
    '''
    def __init__(self, data = None, next = None):
        self.data = data
        self.next = next

class Stack:
    ''' 
    A class of linked lists based stack

    Data attributes
    - self.top = a reference to the first Node
            in the list
    '''

    def __init__(self):
        '''
        Set the stack top to be None
        '''
        pass

    def __str__(self):
        '''
        Create and return a string representation for the stack
        '''
        s = "["
        curr_node = self.top
        while curr_node:
            s += str(curr_node.data)
            curr_node = curr_node.next
            if curr_node != None:
                 s += " -> " 
        s += "]"
        return s


    # ----------- BASIC STACK OPERATIONS --------------#

    def push(self, data):
        """ Push an item to the top of the stack
        """
        # need to create a node with data, then add the node to the stack
        pass

    def pop(self):
        """ Pop the top item from the stack """
        pass

    def is_empty(self):
        """ Return True if stack is empty, False otherwise"""
        pass

    def peek(self):
        """ Return the data at the top of the stack"""
        pass