Insert at End of Doubly Linked List in DSA C - 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 NULL.

beginner

In C, what pointers does a node in a doubly linked list typically contain?

A node contains:
- A data field (e.g., int data)
- A pointer to the next node (struct Node* next)
- A pointer to the previous node (struct Node* prev)

intermediate

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. Setting its previous pointer to the last node keeps the backward link intact, allowing traversal in reverse.

intermediate

What happens if you forget to set the new node's next pointer to NULL when inserting at the end?

The new node might point to some random memory or garbage value, causing undefined behavior when traversing the list. It breaks the list's end marker.

What is the first step when inserting a node at the end of a doubly linked list?

A Set the head pointer

B Traverse to the last node

C Create a new node with the given data

D Set the new node's previous pointer

You must first create the new node before linking it to the list.

If the doubly linked list is empty, what should the new node become?

A The tail node

B The head node

C The previous node

D The middle node

If the list is empty, the new node becomes the head (and tail) of the list.

Which pointer of the last node must be updated to link the new node at the end?

A Head pointer

B Previous pointer

C Data pointer

D Next pointer

The last node's next pointer should point to the new node.

What should the new node's next pointer be set to after insertion at the end?

A NULL

B Head node

C Previous node

D It can be anything

The new node is the last node, so its next pointer must be NULL.

Why is it important to update the new node's previous pointer?

A To link it to the previous node

B To link it to the next node

C To set the head pointer

D To delete the node

The previous pointer links the new node back to the node before it.

Explain the process of inserting a node at the end of a doubly linked list in your own words.

Think about how the new node connects forward and backward.

Describe the structure of a node in a doubly linked list and why each part is important.

Imagine walking through a chain in both directions.

Practice

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

easy

A. To add a new element after the last node

B. To remove the first node

C. To reverse the list

D. To find the middle node

Solution

1. Step 1: Understand the list structure

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

2. Step 2: Purpose of insertion at end

   Inserting at the end means adding a new node after the last node, extending the list.

3. Final Answer:

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

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 happens at the start
• Mixing up singly and doubly linked list operations

2. Which of the following is the correct way to update the 'next' pointer of the last node when inserting a new node at the end in C?

easy

A. last->next = newNode;

B. newNode->next = last;

C. last->prev = newNode;

D. newNode->prev = NULL;

Solution

1. Step 1: Identify the last node's next pointer

   The last node's 'next' pointer should point to the new node to link it at the end.

2. Step 2: Correct pointer assignment

   Assigning 'last->next = newNode;' correctly links the new node after the last node.

3. Final Answer:

   last->next = newNode; -> Option A

4. Quick Check:

   Last node's next points to new node [OK]

Hint: Last node's next points to new node [OK]

Common Mistakes:

• Assigning newNode->next to last (wrong direction)
• Changing last->prev instead of last->next
• Setting newNode->prev to NULL instead of last

3. What will be the output of the following code snippet after inserting a node with value 5 at the end of the doubly linked list 1 <-> 3 <-> 4?

struct Node {
  int data;
  struct Node* next;
  struct Node* prev;
};

void insertEnd(struct Node** head, int val) {
  struct Node* newNode = malloc(sizeof(struct Node));
  newNode->data = val;
  newNode->next = NULL;
  if (*head == NULL) {
    newNode->prev = NULL;
    *head = newNode;
    return;
  }
  struct Node* temp = *head;
  while (temp->next != NULL) {
    temp = temp->next;
  }
  temp->next = newNode;
  newNode->prev = temp;
}

void printList(struct Node* head) {
  while (head != NULL) {
    printf("%d ", head->data);
    head = head->next;
  }
}

int main() {
  struct Node* head = malloc(sizeof(struct Node));
  head->data = 1; head->prev = NULL;
  struct Node* second = malloc(sizeof(struct Node));
  second->data = 3; second->prev = head; head->next = second;
  struct Node* third = malloc(sizeof(struct Node));
  third->data = 4; third->prev = second; second->next = third; third->next = NULL;
  insertEnd(&head, 5);
  printList(head);
  return 0;
}

medium

A. 1 3 4 0

B. 1 3 4 5

C. 1 3 4

D. 5 1 3 4

Solution

1. Step 1: Understand initial list and insertion

   The list is 1 <-> 3 <-> 4. The function inserts 5 at the end by traversing to last node (4) and linking new node after it.

2. Step 2: Trace printList output

   After insertion, list is 1 <-> 3 <-> 4 <-> 5. Printing nodes in order outputs "1 3 4 5 "

3. Final Answer:

   1 3 4 5 -> Option B

4. Quick Check:

   List after insertEnd = 1 3 4 5 [OK]

Hint: Traverse to last, link new node, print all nodes [OK]

Common Mistakes:

• Not setting newNode->prev correctly
• Forgetting to set last node's next to newNode
• Printing before insertion

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

void insertEnd(struct Node** head, int val) {
  struct Node* newNode = malloc(sizeof(struct Node));
  newNode->data = val;
  newNode->next = NULL;
  if (*head == NULL) {
    *head = newNode;
    return;
  }
  struct Node* temp = *head;
  while (temp->next != NULL) {
    temp = temp->next;
  }
  temp->next = newNode;
}

medium

A. Wrong condition in while loop

B. Incorrect malloc size

C. Missing setting newNode->prev pointer

D. Not updating head pointer

Solution

1. Step 1: Check pointer assignments for doubly linked list

   In doubly linked list, newNode->prev must point to the last node (temp) after insertion.

2. Step 2: Identify missing pointer update

   The code sets temp->next = newNode but does not set newNode->prev = temp, breaking backward link.

3. Final Answer:

   Missing setting newNode->prev pointer -> Option C

4. Quick Check:

   newNode->prev must be set [OK]

Hint: Always set newNode->prev to last node [OK]

Common Mistakes:

• Forgetting newNode->prev assignment
• Assuming singly linked list logic applies
• Not handling empty list case

5. Given a doubly linked list with nodes 10 <-> 20 <-> 30, write the sequence of pointer changes when inserting a new node with value 40 at the end. Which of the following correctly describes the final connections?

hard

A. newNode->prev = NULL; newNode->next = NULL; last->next = NULL

B. newNode->next = last; last->prev = newNode; newNode->prev = NULL

C. last->prev = newNode; newNode->next = last; newNode->prev = NULL

D. last->next = newNode; newNode->prev = last; newNode->next = NULL

Solution

1. Step 1: Identify last node and new node pointers

   Last node is 30. New node 40 should be linked after 30 with next NULL.

2. Step 2: Correct pointer assignments

   Set last->next = newNode to link forward, newNode->prev = last for backward link, and newNode->next = NULL as it is new end.

3. Final Answer:

   last->next = newNode; newNode->prev = last; newNode->next = NULL -> Option D

4. Quick Check:

   New node linked after last with proper prev and next [OK]

Hint: New node's prev = last; last's next = new node; new node's next = NULL [OK]

Common Mistakes:

• Reversing prev and next pointers
• Setting newNode->prev to NULL incorrectly
• Not setting last->next to newNode