CODEWITHSUNDEEP
pythonpython-3.x
vkaul11
vkaul11

Reputation: 4214

unable to access next node for Linked List while reversing a Linked List in python

I am a bit new to python and I have seen the correct solutions to the reversing the linkedlist problem but I wanted to know why my solution does not work. In particular, reverse function stays inside the while loop for the code below because of "new_list.head.next=prev" line 

class Node:
    def __init__(self, value):
        self.value = value
        self.next = None

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

    def append(self, value):
        if self.head is None:
            self.head = Node(value)
            return

        node = self.head
        while node.next:
            node = node.next

        node.next = Node(value)

    def __iter__(self):
        node = self.head
        while node:
            yield node.value
            node = node.next

    def __repr__(self):
        return str([v for v in self])

def reverse(linked_list):
    new_list = LinkedList()
    if linked_list is None:
        return new_list
    node = linked_list.head
    new_list.head = node 
    while node.next:
        prev = node
        node = node.next
        new_list.head = node
        new_list.head.next = prev
    return new_list   

if __name__ == "__main__":
    a = LinkedList()
    b = [1,2,3,4,5]
    for item in b:
        a.append(item)
    print a
    c = reverse(a) 
    print c

Upvotes: 1

Views: 82

Answers (3)

Error - Syntactical Remorse
Error - Syntactical Remorse

Reputation: 7887

If you tag your question with Python3 please make sure it runs in python 3.

The reason is because you are mixing up points and creating an infinite loop. Print the value and it may help you find the bug. I am going to use the values to point out the issue.

    while node.next:
        # First node that comes in value = 1
        print(node.value) #
        prev = node # prev = 1
        node = node.next # node = 2
        new_list.head = node # You are setting the head = 2 (i.e. head = node.next)
        new_list.head.next = prev # You are setting next of head = 1 (i.e. head.next = node.next.next)
        # however this also set node.next.next = 1
        # So going into the next loop: node.value = 2 and node.next.value = 1

Because of this pointer confusion you are forever looping between your first and second node.

Upvotes: 2

Mark
Mark

Reputation: 92440

It's a little easier to reason about this (at least to me) if you think about two references:

• One to the remaining part of the original list you haven't seen
• One to the head of the new list

At each iteration move the remaining up and set the old remaining to the head of the new list. Order is important here — as you've seen it's easy to accidentally change next on two different variables that are pointing the same node if you're not careful:

def reverse(linked_list):
    new_list = LinkedList()

    if linked_list is None:
        return new_list

    remaining = linked_list.head

    while remaining:
        prev_head = new_list.head       # the old head becomes the first link
        new_list.head = remaining       # new head becomese the first remaining
        remaining = remaining.next      # move remaing one up the chain
        new_list.head.next = prev_head  # point the new head to the previous   
    return new_list

Upvotes: 1

sanyassh
sanyassh

Reputation: 8520

This is how your reverse can look:

def reverse(linked_list):
    new_list = LinkedList()
    if linked_list is None:
        return new_list
    node = linked_list.head
    new_list.head = Node(node.value)
    while node.next:
        node = node.next
        prev_head = new_list.head
        new_list.head = Node(node.value)
        new_list.head.next = prev_head
    return new_list

With it I got desired output of print c: [5, 4, 3, 2, 1]

General advise: create new Node instead of assignment to node in initial list.

Upvotes: 2

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