How to create a listnode in python

I am trying to solve a linked list coding challenge in python. And I have given only following class to create a linked list

# Definition for singly-linked list.
class ListNode[object]:
    def __init__[self, x]:
        self.val = x
        self.next = None

I can create a linked list something like this

x = ListNode[1]
x.next = ListNode[4]
x.next.next = ListNode[5]

however, How do I create iterativly [inside a for loop]

asked May 4, 2019 at 23:14

Gaurang ShahGaurang Shah

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You need two "pointers" that memorize the head and the tail of your list. The head is initialized once. You will eventually use it to access the whole list. The tail changes every time you add another node:

data = [5, 1, 7, 96]
tail = head = ListNode[data[0]]
for x in data[1:]:
    tail.next = ListNode[x] # Create and add another node
    tail = tail.next # Move the tail pointer

answered May 5, 2019 at 3:23

You can do something like:

arr = [1,4,5]
for i in arr:
    x = ListNode[i]
    x = x.next

But now x will become None. As there's nothing else to keep track, you can't print the elements.

You can verify this by printing the values of x.val in between the loop:

arr = [1,4,5]
for i in arr:
    x = ListNode[i]
    print[x.val]
    x = x.next

output:

1
4
5

answered May 4, 2019 at 23:21

R4444R4444

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1

You can add a next argument to the constructor:

class ListNode[object]:
    def __init__[self, x, next=None]:
        self.x = x
        self.next = next

head = None
for x in [5, 1, 7, 96]:
    head = ListNode[x, next=head]

# Linked list looks like:
# [96] -> [ 7] -> [ 1] -> [ 5] -> None

answered May 4, 2019 at 23:40

Brian RodriguezBrian Rodriguez

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Educative Answers Team

A linked list is a data structure made of a chain of node objects. Each node contains a value and a pointer to the next node in the chain.

Linked lists are preferred over arrays due to their dynamic size and ease of insertion and deletion properties.

The head pointer points to the first node, and the last element of the list points to null. When the list is empty, the head pointer points to null.

Linked lists in Python

Original Python does not ship with a built-in linked list data structure like the one seen in Java.

Let’s see how we can create our own implementation of a standard class-based singly linked list in Python.

1. Start with a single node

Let’s start with a single node since linking several nodes gives us a complete list. For this, we make a Node class that holds some data and a single pointer next, that will be used to point to the next Node type object in the Linked List.

# A single node of a singly linked list
class Node:
  # constructor
  def __init__[self, data, next=None]: 
    self.data = data
    self.next = next

# Creating a single node
first = Node[3]
print[first.data]

2. Join nodes to get a linked list

The next step is to join multiple single nodes containing data using the next pointers, and have a single head pointer pointing to a complete instance of a Linked List.

Using the head pointer, we will be able to traverse the whole list, even perform all kinds of list manipulations while we are at it.

For this, we create a LinkedList class with a single head pointer:

# A single node of a singly linked list
class Node:
  # constructor
  def __init__[self, data = None, next=None]: 
    self.data = data
    self.next = next

# A Linked List class with a single head node
class LinkedList:
  def __init__[self]:  
    self.head = None

# Linked List with a single node
LL = LinkedList[]
LL.head = Node[3]
print[LL.head.data]

3. Add required methods to the LinkedList class

Last but not least, we can add various linked list manipulation methods in our implementation.

Let’s have a look at the insertion and print methods for our LinkedList implementation below:

# A single node of a singly linked list
class Node:
  # constructor
  def __init__[self, data = None, next=None]: 
    self.data = data
    self.next = next

# A Linked List class with a single head node
class LinkedList:
  def __init__[self]:  
    self.head = None
  
  # insertion method for the linked list
  def insert[self, data]:
    newNode = Node[data]
    if[self.head]:
      current = self.head
      while[current.next]:
        current = current.next
      current.next = newNode
    else:
      self.head = newNode
  
  # print method for the linked list
  def printLL[self]:
    current = self.head
    while[current]:
      print[current.data]
      current = current.next

# Singly Linked List with insertion and print methods
LL = LinkedList[]
LL.insert[3]
LL.insert[4]
LL.insert[5]
LL.printLL[]

RELATED TAGS

data structures

python

linked list

node

classes

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