class SLinkedList:
    # an instance of this class is a Singly-Linked List object
    # it has reference to the first node in the list
    def __init__(self):
        self.head = None
        self.size = 0
    def add(self,item):
        # adds an item at the start of the list
        new_node = SLinkedListNode(item,None)
        new_node.setNext(self.head)
        self.head = new_node
        self.size = self.size + 1
    def append(self,item):
        # adds an item at the end of the list
        new_node = SLinkedListNode(item,None)
        current = self.head # Start the traversal
        if self.size == 0: # check if list is empty
            self.add(item)
        else:
            while (current.getNext()!=None):
                current= current.getNext() # traversing the list
            current.setNext(new_node)
            self.size = self.size +1


    def insert(self,pos,item):
        # inserts the item at pos
        # pos should be a positive number of type int
        # TO DO: write assert statement that tests if pos is int
        # TO DO: write assert statement that tests that pos is not negative

        # TO DO: COMPLETE THE METHOD
        assert isinstance(pos,int), 'pos has to be int'
        assert pos >= 0, 'pos has to be positive'

        if pos == self.size:
            self.append(item)
        elif pos == 0:
            self.add(item)
        else:
            current = self.head
            for i in range(pos-1):
                current = current.getNext()

            new_node = SLinkedListNode(item,current.getNext())
            current.setNext(new_node)


    def remove(self,item):
        # remove the node containing the item from the list
        if self.size == 0:
            raise Exception('List is Empty')
        current = self.head
        previous = None
        found = False
        while current != None and not found:
            if current.getData() == item:
                found = True
            else:
                previous = current
                current = current.getNext()
        if not found:
            raise Exception('Item not in list')
        else:
            if previous == None: # the item is in the first node of the list
                self.head = current.getNext()
            else: # item is not in the first node
                previous.setNext(current.getNext())
            self.size = self.size -1
    def index(self,item):
        # finds the location of the item in the list
        if self.size == 0:
            raise Exception('List is empty')
        position = 0
        found = False
        current = self.head
        while current != None and not found:
            if current.getData() == item:
                found = True
            else:
                current = current.getNext()
                position = position + 1
        if found:
            return position
        else:
            return 'Item not found'
    def pop(self):
        # removes the node from the end of the list and returns the item
        if self.size == 0:
            raise Exception('List is empty')
        current = self.head
        previous = None
        while current.getNext() != None:
            previous = current
            current = current.getNext()
        if previous == None:
            self.head = None
        else:
            previous.setNext(None)
        self.size = self.size -1
        return current.getData()
    def __str__(self):
        # returns a string representation of the list
        current = self.head
        string = ''
        while current != None:
            string = string + str(current.getData())+'->'
            current = current.getNext()
        return string
    def getSize(self):
        return self.size