Insert at End of Doubly Linked List in DSA Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a doubly linked list?

A doubly linked list is a chain of nodes where each node has three parts: data, a pointer to the next node, and a pointer to the previous node. This allows moving forward and backward through the list.

beginner

What steps are needed to insert a new node at the end of a doubly linked list?

1. Create a new node with the given data.
2. If the list is empty, make the new node the head.
3. Otherwise, traverse to the last node.
4. Set the last node's next pointer to the new node.
5. Set the new node's previous pointer to the last node.
6. Set the new node's next pointer to None.

beginner

Why do we need to update the previous pointer of the new node when inserting at the end?

Because the new node must know which node comes before it to maintain the doubly linked list structure, allowing backward traversal.

intermediate

What happens if you forget to set the new node's next pointer to None when inserting at the end?

The list may have incorrect links, possibly causing traversal errors or infinite loops because the new node might point to an unexpected node.

beginner

Show the Python code snippet to insert a node at the end of a doubly linked list.

class Node:<br>    def __init__(self, data):<br>        self.data = data<br>        self.next = None<br>        self.prev = None<br><br>class DoublyLinkedList:<br>    def __init__(self):<br>        self.head = None<br><br>    def insert_at_end(self, data):<br>        new_node = Node(data)<br>        if not self.head:<br>            self.head = new_node<br>            return<br>        last = self.head<br>        while last.next:<br>            last = last.next<br>        last.next = new_node<br>        new_node.prev = last<br>        new_node.next = None

What pointer of the new node must be set when inserting at the end of a doubly linked list?

A prev pointer to None

B next pointer to the head node

C prev pointer to the last node

D next pointer to the previous node

The new node's prev pointer must point to the last node to maintain the backward link.

If the doubly linked list is empty, what should the insert_at_end function do?

A Set the new node as the head

B Return without inserting

C Set the new node's next to head

D Set the new node's prev to head

When the list is empty, the new node becomes the head of the list.

Which traversal is needed to insert at the end of a doubly linked list?

A Traverse from tail to head

B Traverse only one node

C No traversal needed

D Traverse from head to last node

We must traverse from the head to find the last node to insert after it.

What happens if you forget to update the last node's next pointer when inserting at the end?

A The new node is not linked to the list

B The list becomes circular

C The new node becomes head

D The list deletes the last node

Without updating last node's next pointer, the new node is not connected to the list.

What is the time complexity of inserting at the end of a doubly linked list without a tail pointer?

A O(n^2)

B O(n)

C O(log n)

D O(1)

Without a tail pointer, we must traverse the list to find the end, which takes O(n) time.

Explain step-by-step how to insert a new node at the end of a doubly linked list.

Think about how the pointers connect the nodes forward and backward.

Write a simple Python function to insert a node at the end of a doubly linked list and explain how it works.

Focus on pointer updates and empty list handling.

Practice

1. What is the main purpose of inserting a node at the end of a doubly linked list?

easy

A. To reverse the list

B. To add a new element after the last node

C. To remove the first node

D. To find the middle node

Solution

1. Step 1: Understand the structure of a doubly linked list

   A doubly linked list has nodes connected forward and backward, allowing traversal in both directions.

2. Step 2: Purpose of insertion at the end

   Inserting at the end means adding a new node after the current last node, updating pointers accordingly.

3. Final Answer:

   To add a new element after the last node -> Option B

4. Quick Check:

   Insert at end = Add after last node [OK]

Hint: Insert at end means add after last node [OK]

Common Mistakes:

• Confusing insertion at end with deletion
• Thinking insertion reverses the list
• Assuming insertion happens at the start

2. Which of the following is the correct way to update the pointers when inserting a new node at the end of a doubly linked list in Python?

easy

A. new_node.prev = null; new_node.next = head; head.prev = new_node; head = new_node

B. new_node.next = head; new_node.prev = null; head.prev = new_node; head = new_node

C. new_node.next = null; new_node.prev = tail; tail.next = new_node; tail = new_node

D. new_node.prev = tail; new_node.next = tail; tail.next = new_node; tail = new_node

Solution

1. Step 1: Identify pointer updates for insertion at end

   The new node's next pointer should be null because it will be the last node. Its prev pointer should point to the current tail.

2. Step 2: Update the tail's next pointer and move tail

   The current tail's next pointer should point to the new node, then update tail to the new node.

3. Final Answer:

   new_node.next = null; new_node.prev = tail; tail.next = new_node; tail = new_node -> Option C

4. Quick Check:

   New node next = null, prev = tail [OK]

Hint: New node next is null, prev points to tail [OK]

Common Mistakes:

• Setting new_node.next to head instead of null
• Not updating tail pointer after insertion
• Confusing prev and next pointers

3. What will be the printed output of the doubly linked list after executing the following code?

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

class DoublyLinkedList:
    def __init__(self):
        self.head = None
        self.tail = None

    def insert_end(self, data):
        new_node = Node(data)
        if not self.head:
            self.head = self.tail = new_node
        else:
            self.tail.next = new_node
            new_node.prev = self.tail
            self.tail = new_node

    def print_list(self):
        current = self.head
        result = []
        while current:
            result.append(current.data)
            current = current.next
        print(' -> '.join(map(str, result)) + ' -> null')

dll = DoublyLinkedList()
dll.insert_end(10)
dll.insert_end(20)
dll.insert_end(30)
dll.print_list()

medium

A. 10 -> 20 -> 30 -> null

B. 30 -> 20 -> 10 -> null

C. 10 -> 20 -> null

D. null

Solution

1. Step 1: Insert nodes 10, 20, 30 at the end

   Each insert_end call adds a new node at the tail, linking nodes in order: 10, then 20, then 30.

2. Step 2: Print the list from head to tail

   print_list traverses from head, collecting data: 10, 20, 30, then prints with arrows and 'null' at end.

3. Final Answer:

   10 -> 20 -> 30 -> null -> Option A

4. Quick Check:

   Insert end adds at tail, print from head [OK]

Hint: Insert end adds at tail; print from head shows order [OK]

Common Mistakes:

• Confusing print order as reversed
• Missing last node in output
• Printing null instead of 'null'

4. Identify the error in the following code snippet that inserts a node at the end of a doubly linked list:

def insert_end(self, data):
    new_node = Node(data)
    if self.head is None:
        self.head = new_node
    else:
        self.tail.next = new_node
        new_node.prev = self.tail
        self.tail = new_node

medium

A. new_node.next should point to head

B. new_node.prev should be null when list is empty

C. self.head should be updated after insertion at end

D. Missing update of tail when list is empty

Solution

1. Step 1: Check tail update when list is empty

   When the list is empty, only head is set to new_node, but tail is not updated, so tail remains null.

2. Step 2: Correct tail assignment

   Tail must also be set to new_node when list is empty to keep tail pointer valid.

3. Final Answer:

   Missing update of tail when list is empty -> Option D

4. Quick Check:

   Empty list needs tail = new_node too [OK]

Hint: Update tail when list is empty [OK]

Common Mistakes:

• Not updating tail on empty list
• Setting new_node.next incorrectly
• Confusing head and tail updates

5. Given a doubly linked list with nodes containing values [5, 15, 25], you want to insert a new node with value 35 at the end. After insertion, what will be the correct sequence of node values when traversing from head to tail and tail to head respectively?

hard

A. 5 -> 15 -> 25 -> 35 -> null and 35 -> 25 -> 15 -> 5 -> null

B. 35 -> 5 -> 15 -> 25 -> null and 25 -> 15 -> 5 -> 35 -> null

C. 5 -> 15 -> 25 -> null and 35 -> null

D. 5 -> 15 -> 25 -> 35 -> null and 5 -> 15 -> 25 -> 35 -> null

Solution

1. Step 1: Insert 35 at the end of the list

   The new node with value 35 is added after the current tail (25), updating pointers accordingly.

2. Step 2: Traverse from head to tail and tail to head

   From head: 5 -> 15 -> 25 -> 35 -> null; From tail: 35 -> 25 -> 15 -> 5 -> null.

3. Final Answer:

   5 -> 15 -> 25 -> 35 -> null and 35 -> 25 -> 15 -> 5 -> null -> Option A

4. Quick Check:

   Forward and backward traversal show correct order [OK]

Hint: Forward order adds new node at end; backward reverses [OK]

Common Mistakes:

• Mixing forward and backward traversal order
• Forgetting to update prev pointer of new node
• Assuming new node is added at head