""" A list implemetation of Stack
    CSCI 204, fall 2017
    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, init_list = None):
        '''
        Given a python list, creates a linked list. 
        Initializes self.head as the start of the list

        Input:
        - initList: a python list
        '''
        self.top = None

        if init_list is not None:
            self._create_list(init_list)

    def _create_list(self, init_list):
        ''' Given a python list, create a linked list'''
        for index in range(len(init_list)-1, -1, -1):
            self.top = Node(init_list[index], self.top)

    def __repr__(self):
        linkedList = ""
        curr_node = self.top
        while curr_node:
            linkedList += str(curr_node.data) + " -> " 
            curr_node = curr_node.next
        return linked_list

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

    def push(self, data):
        """ Push a itemm to the top of the stack """
        node = Node(data)
        node.next = self.top
        self.top = node

    def pop(self):
        """ Pop the top item from the stack """
        assert not self.is_empty(), "Cannot pop an empty stack"
        data = self.top.data
        self.top = self.top.next
        return data

    def is_empty(self):
        """ Return True if stack is empty, False otherwise"""
        return self.top == None

    def peek(self):
        """ Return the data at the top of the stack"""
        assert not self.is_empty(), "Cannot peek at an empty stack"
        return self.top.data