import node

class LLStack(object):
    
    """An implementation of the Stack ADT using a linked list."""
    
    def __init__(self):
        """Create a new, empty LLStack."""
        #Empty LLStack is created with head as None & size as 0.
        self._head = None
        self._size = 0
        
    def push(self, item):
        """Put item on top of the LLStack."""
        #The new node is created and the current head is linked to it.
        #The head is reassigned as the new node & size is increased by 1.
        temp = node.Node(item)
        temp.next = self._head
        self._head = temp
        self._size += 1   
        
    def pop(self):
        """Return and remove the item on top of the LLStack."""
        #If the size is not 0, the current head is stored in a temp variable 
        #and the head is reassigned as the node linked to the old head. Size is
        #decreased by 1. The data from the temp variable is returned.
        if self._size != 0:
            temp_node = self._head
            aelf._head = self._head.next
            self._size -= 1
            return temp_node.data
            
    def peek(self):
        """Return the item on top of the LLStack without removing it."""
        #Data of the head is returned if size is not 0.
        if self._size != 0:
            return self._head.data
            
    def empty(self):
        """Whether or not the LLStack is empty."""
        #Returns True when the head is None. Otherwise is returns False.
        return self._head == None
    
    def size(self):
        """Return the number of items in the LLStack."""
        #Returns size.
        return self._size