Insert at Specific Position in Doubly Linked List in DSA Go - Execution Trace

Concept Flow - Insert at Specific Position in Doubly Linked List

Create new node with value

↓

Check if position is 1

↓

Insert at head

↓

Traverse to position-1 node

↓

Adjust pointers: new node prev and next

↓

Adjust neighbors' pointers

↓

Insertion done

Create a new node, then either insert at head if position is 1, or traverse to the node before the position and adjust pointers to insert the new node.

Execution Sample

DSA Go

func InsertAtPosition(head *Node, data int, pos int) *Node {
  newNode := &Node{data: data}
  if pos == 1 {
    newNode.next = head
    if head != nil { head.prev = newNode }
    return newNode
  }
  current := head
  for i := 1; i < pos-1 && current != nil; i++ {
    current = current.next
  }
  if current == nil { return head }
  newNode.next = current.next
  if current.next != nil { current.next.prev = newNode }
  current.next = newNode
  newNode.prev = current
  return head
}

This code inserts a new node with given data at the specified position in a doubly linked list.

Execution Table

Step	Operation	Nodes in List	Pointer Changes	Visual State
1	Create new node with data=5	1 -> 2 -> 3 -> null	newNode created	1 <-> 2 <-> 3 <-> null
2	Position=3, start at head	1 -> 2 -> 3 -> null	current = head (node 1)	1 <-> 2 <-> 3 <-> null
3	Traverse to position-1 (2)	1 -> 2 -> 3 -> null	current moves to node 2	1 <-> 2 <-> 3 <-> null
4	Adjust newNode pointers	1 -> 2 -> 3 -> null	newNode.prev = 2, newNode.next = 3	1 <-> 2 <-> 5 <-> 3 <-> null
5	Adjust neighbors' pointers	1 -> 2 -> 5 -> 3 -> null	2.next = newNode, 3.prev = newNode	1 <-> 2 <-> 5 <-> 3 <-> null
6	Insertion done	1 -> 2 -> 5 -> 3 -> null	head unchanged	1 <-> 2 <-> 5 <-> 3 <-> null
7	Exit	Insertion complete	No changes	1 <-> 2 <-> 5 <-> 3 <-> null

💡 Reached position 3, inserted new node, pointers updated, insertion complete

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	Final
head	Node 1	Node 1	Node 1	Node 1	Node 1	Node 1
newNode	nil	Node 5	Node 5	Node 5	Node 5	Node 5
current	Node 1	Node 1	Node 2	Node 2	Node 2	Node 2
position	3	3	3	3	3	3

Key Moments - 3 Insights

Why do we traverse to position-1 node instead of the position node?

Because we need to insert the new node after the position-1 node, so we adjust pointers around it. See execution_table step 3 where current moves to node 2 before insertion.

What happens if position is 1?

The new node becomes the new head. Its next points to old head, and old head's prev points back to new node. This is handled separately before traversal, as shown in concept_flow.

Why do we check if current.next is nil before adjusting its prev pointer?

Because if current.next is nil, it means we are inserting at the end, so no next node exists to update. This prevents null pointer errors, as shown in execution_table step 4.

Visual Quiz - 3 Questions

Look at the execution_table, what is the 'current' node after step 3?

A Node 2

B Node 1

C Node 3

D Node 5

💡 Hint

Check the 'Pointer Changes' column in step 3 where current moves to node 2.

At which step is the new node's prev pointer set?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look at step 4 'Adjust newNode pointers' where newNode.prev is assigned.

If position was 1, which step would be skipped?

A Step 1

B Step 3

C Step 4

D Step 5

💡 Hint

Refer to concept_flow where insertion at head skips traversal.

Concept Snapshot

Insert at Specific Position in Doubly Linked List:
- Create new node
- If position=1, insert at head
- Else, traverse to position-1 node
- Adjust new node's prev and next pointers
- Update neighbors' pointers
- Return head (new or old)
Pointers must be carefully updated to maintain list integrity.

Full Transcript

To insert a node at a specific position in a doubly linked list, first create the new node. If the position is 1, insert the new node at the head by adjusting its next pointer to the old head and updating the old head's prev pointer. Otherwise, start from the head and traverse to the node just before the desired position. Then, adjust the new node's prev pointer to this node and its next pointer to the next node. Update the neighbors' pointers accordingly to include the new node. Finally, return the head of the list. This process ensures the doubly linked list remains connected correctly after insertion.

Practice

1. What does inserting a node at a specific position in a doubly linked list involve?

easy

A. Only changing the head pointer of the list

B. Adjusting the previous and next pointers of neighboring nodes to include the new node

C. Replacing the node at that position without changing pointers

D. Appending the node at the end regardless of position

Solution

1. Step 1: Understand doubly linked list structure

   Each node has pointers to both previous and next nodes, so insertion requires updating these pointers.

2. Step 2: Insert node at specific position

   To insert, update the previous node's next pointer and the next node's previous pointer to point to the new node.

3. Final Answer:

   Adjusting the previous and next pointers of neighboring nodes to include the new node -> Option B

4. Quick Check:

   Pointer updates = A [OK]

Hint: Remember both previous and next pointers must be updated [OK]

Common Mistakes:

• Only updating one pointer and ignoring the other
• Not handling edge cases like insertion at head or tail
• Replacing node data instead of inserting new node

2. Which of the following Go code snippets correctly declares a doubly linked list node struct for insertion?

easy

A. type Node struct { data int; prev *Node; next *Node }

B. type Node struct { data int; next *Node }

C. type Node struct { data int; prev int; next int }

D. type Node struct { data int; prev Node; next Node }

Solution

1. Step 1: Check struct fields for doubly linked list

   A doubly linked list node must have pointers to previous and next nodes, both of type *Node.

2. Step 2: Validate Go syntax

   type Node struct { data int; prev *Node; next *Node } correctly uses pointers (*Node) for prev and next fields, matching doubly linked list requirements.

3. Final Answer:

   type Node struct { data int; prev *Node; next *Node } -> Option A

4. Quick Check:

   Pointers for prev and next = A [OK]

Hint: Use *Node type for prev and next pointers in struct [OK]

Common Mistakes:

• Using non-pointer types for prev and next
• Omitting prev pointer for doubly linked list
• Using incorrect field types like int for pointers

3. What will be the output of the doubly linked list after inserting a node with value 25 at position 3 in the list 10 <-> 20 <-> 30 <-> 40?

medium

A. 10 <-> 20 <-> 25 <-> 30 <-> 40

B. 10 <-> 25 <-> 20 <-> 30 <-> 40

C. 25 <-> 10 <-> 20 <-> 30 <-> 40

D. 10 <-> 20 <-> 30 <-> 40 <-> 25

Solution

1. Step 1: Identify position 3 in the list

   Positions: 1=10, 2=20, 3=30, 4=40. Inserting at position 3 means before node with value 30.

2. Step 2: Insert node with value 25 at position 3

   New node 25 goes between 20 and 30, updating pointers accordingly.

3. Final Answer:

   10 <-> 20 <-> 25 <-> 30 <-> 40 -> Option A

4. Quick Check:

   Insert before 3rd node = D [OK]

Hint: Insert before node currently at that position [OK]

Common Mistakes:

• Inserting after the node at position instead of before
• Appending at end instead of correct position
• Confusing position indexing starting at 0

4. Given the following Go code snippet for insertion, what is the error?

func InsertAtPosition(head *Node, data int, pos int) *Node {
    newNode := &Node{data: data}
    if pos == 1 {
        newNode.next = head
        head.prev = newNode
        head = newNode
        return head
    }
    temp := head
    for i := 1; i < pos-1; i++ {
        temp = temp.next
    }
    newNode.next = temp.next
    temp.next.prev = newNode
    temp.next = newNode
    newNode.prev = temp
    return head
}

medium

A. Incorrect assignment of newNode.prev

B. Missing creation of newNode

C. No check if temp or temp.next is nil before accessing pointers

D. Returning newNode instead of head

Solution

1. Step 1: Analyze pointer usage in loop and insertion

   The loop moves temp to position before insertion. If pos is beyond list length, temp or temp.next can be nil.

2. Step 2: Identify missing nil checks

   Accessing temp.next.prev without checking if temp.next is nil can cause runtime panic.

3. Final Answer:

   No check if temp or temp.next is nil before accessing pointers -> Option C

4. Quick Check:

   Missing nil checks cause runtime errors [OK]

Hint: Always check for nil before accessing next or prev pointers [OK]

Common Mistakes:

• Ignoring nil pointer checks causing crashes
• Incorrect pointer assignments order
• Returning wrong node pointer

5. You have a doubly linked list: 5 <-> 15 <-> 25 <-> 35. You want to insert a node with value 20 at position 3. After insertion, what will be the correct order of nodes and how should pointers be updated?

hard

A. 5 <-> 20 <-> 15 <-> 25 <-> 35; update 5.next to 20, 20.prev to 5, 20.next to 15, 15.prev to 20

B. 5 <-> 15 <-> 25 <-> 20 <-> 35; update 25.next to 20, 20.prev to 25, 20.next to 35, 35.prev to 20

C. 20 <-> 5 <-> 15 <-> 25 <-> 35; update head to 20, 20.next to 5, 5.prev to 20

D. 5 <-> 15 <-> 20 <-> 25 <-> 35; update 15.next to 20, 20.prev to 15, 20.next to 25, 25.prev to 20

Solution

1. Step 1: Determine correct insertion position

   Position 3 means node with value 20 goes between nodes 15 (pos 2) and 25 (pos 3).

2. Step 2: Update pointers for insertion

   Set 15.next to new node 20, 20.prev to 15, 20.next to 25, and 25.prev to 20 to maintain list integrity.

3. Final Answer:

   5 <-> 15 <-> 20 <-> 25 <-> 35; update 15.next to 20, 20.prev to 15, 20.next to 25, 25.prev to 20 -> Option D

4. Quick Check:

   Insert between 15 and 25 with pointer updates = C [OK]

Hint: Insert between nodes and update four pointers correctly [OK]

Common Mistakes:

• Swapping nodes or inserting at wrong position
• Forgetting to update prev pointers
• Incorrectly updating next pointers causing broken links