Reversing Linked List and displaying it like original

Question

I successfully implemented a single linked list with a Display function that prints the list elements. I created an iterative reverse function but the list displayed is missing the last element and displaying None instead.

I went over my algorithm many times. Is there anything I am missing?

Thanks in advance.

class node(object):
    def __init__(self, data=None):
        self.data = data
        self.next = None

class LinkedList(object):
    def __init__(self, head=None):
        self.head = node()

    # append to list
    def append(self, data):
        new_node = node(data)
        current = self.head  # head of the list
        while current.next != None:  # while not last node
            current = current.next  # traverse
        current.next = new_node  # append

    def display(self):
        list = []
        current = self.head
        while current.next != None:
            current = current.next
            list.append(current.data)
        print(list)
        return

    def reverse(self): 
        current = self.head
        prev = None
        while current:
            next_ = current.next
            current.next = prev
            prev = current
            current = next_
        self.head = prev

Test Case:

list = LinkedList()
list.append(0)
list.append(1)
list.append(2)
list.append(3)
list.append(4)
list.display()
list.reverse()
list.display()

Output:

[0, 1, 2, 3, 4]
[3, 2, 1, 0, None]

Answer 1

The problem is that your linked list starts off with a blank head because of your constructors for node and linked list.

class node(object):
    def __init__(self, data=None):
        self.data = data
        self.next = None
class LinkedList(object):
    def __init__(self, head=None):
        self.head = node()

If you notice when you create a new LinkedList object you will get a head that has no data in it, and you compensate in your print statement/appends by getting self.head.next first:

current = self.head  # head of the list
    while current.next != None:  # while not last node

So that means when you set self.head in your reverse class at the very end you are setting the head to a non-blank head and you skip it in your print.

To compensate for this you need to create a new blank head and set next to prev:

    def reverse(self):
    current = self.head.next
    prev = None
    while current:
        next_ = current.next
        current.next = prev

        prev = current
        current = next_

    #We create a new blank head and set the next to our valid list
    newHead = node()
    newHead.next = prev
    self.head = newHead

Output is

[0, 1, 2, 3, 4]
[4, 3, 2, 1, 0]