Insert at End of Doubly Linked List in DSA C - Execution Trace

Concept Flow - Insert at End of Doubly Linked List

Create new node with data

↓

Is list empty?

↓

Set head = new

↓

Traverse to last node

↓

Set last.next = new

↓

Set new.prev = last

↓

Done

Create a new node, check if list is empty, if yes set head to new node; else traverse to last node, link new node at end with proper prev and next pointers.

Execution Sample

DSA C

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

void insertEnd(struct Node** head, int val) {
  // Insert val at end
}

Insert a new node with given value at the end of a doubly linked list.

Execution Table

Step	Operation	Nodes in List	Pointer Changes	Visual State
1	Create new node with data=10	Empty	new_node created with data=10, prev=∅, next=∅	∅ (empty list)
2	Check if list is empty	Empty	head == ∅ (true)	∅ (empty list)
3	Set head = new_node	1 node	head → new_node	┌────────┐ │ data:10│ │ prev:∅ │ │ next:∅ │ └────────┘ head
4	Create new node with data=20	1 node	new_node created with data=20, prev=∅, next=∅	┌────────┐ ┌────────┐ │ data:10│ │ data:20│ │ prev:∅ │ │ prev:∅ │ │ next:∅ │ │ next:∅ │ └────────┘ └────────┘ head
5	Check if list is empty	1 node	head != ∅ (false)	┌────────┐ │ data:10│ │ prev:∅ │ │ next:∅ │ └────────┘ head
6	Traverse to last node	1 node	current = head (data=10), current.next == ∅ (stop)	┌────────┐ │ data:10│ │ prev:∅ │ │ next:∅ │ └────────┘ head, current
7	Set last.next = new_node	2 nodes	current.next → new_node (data=20)	┌────────┐ ┌────────┐ │ data:10│ │ data:20│ │ prev:∅ │ │ prev:∅ │ │ next:──┼──→ │ next:∅ │ └────────┘ └────────┘ head current.next
8	Set new_node.prev = last	2 nodes	new_node.prev → current (data=10)	┌────────┐ ┌────────┐ │ data:10│ │ data:20│ │ prev:∅ │ │ prev:──┼──→ head │ next:──┼──→ │ next:∅ │ └────────┘ └────────┘ head new_node
9	Done	2 nodes	List updated with new tail	┌────────┐ ┌────────┐ │ data:10│ │ data:20│ │ prev:∅ │←───┤ prev:──┼──→ ∅ │ next:──┼──→ │ next:∅ │ └────────┘ └────────┘ head tail

💡 Insertion complete; new node linked at end with correct prev and next pointers.

Variable Tracker

Variable	Start	After Step 1	After Step 3	After Step 4	After Step 6	After Step 7	After Step 8	Final
head	∅	∅	→ node(10)	→ node(10)	→ node(10)	→ node(10)	→ node(10)	→ node(10)
new_node	∅	node(10)	node(10)	node(20)	node(20)	node(20)	node(20)	node(20)
current	∅	∅	∅	∅	node(10)	node(10)	node(10)	node(10)

Key Moments - 3 Insights

Why do we check if the list is empty before inserting?

Because if the list is empty (head == ∅), we must set head to the new node directly (see execution_table step 2 and 3). Otherwise, traversal to the last node is needed.

Why do we set new_node.prev to the last node?

In a doubly linked list, each node points back to its previous node. Setting new_node.prev to last node maintains this backward link (see execution_table step 8).

How do we know where to insert the new node?

We traverse from head until current.next is ∅, meaning current is the last node. Then we link new_node after current (see execution_table step 6 and 7).

Visual Quiz - 3 Questions

Look at the execution_table, what is the state of head after step 3?

A head is still empty (∅)

B head points to node with data 10

C head points to node with data 20

D head points to new_node with data 20

💡 Hint

Check the 'Pointer Changes' and 'Visual State' columns at step 3 in execution_table.

At which step does the traversal to the last node stop?

A Step 4

B Step 5

C Step 6

D Step 7

💡 Hint

Look at the 'Operation' and 'Pointer Changes' columns in execution_table to find when current.next == ∅.

If the list was empty, which step would be skipped?

A Step 6 (traverse to last node)

B Step 3 (set head = new_node)

C Step 2 (check if list is empty)

D Step 8 (set new_node.prev = last)

💡 Hint

Refer to the concept_flow and execution_table steps for empty list insertion.

Concept Snapshot

Insert at End of Doubly Linked List:
- Create new node with data
- If list empty, set head = new node
- Else traverse to last node
- Set last.next = new node
- Set new_node.prev = last node
- New node's next = ∅ (null)
- List size increases by 1

Full Transcript

To insert at the end of a doubly linked list, first create a new node with the given data. Check if the list is empty by seeing if head is null. If empty, set head to the new node. If not, start from head and move through nodes until reaching the last node (where next is null). Then link the new node after the last node by setting last.next to new node and new_node.prev to last node. The new node's next pointer is set to null. This maintains the doubly linked structure with forward and backward links. The list now has one more node at the end.