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

Why do we need to update both next and previous pointers when inserting a node in a doubly linked list?

Because each node points to both its next and previous nodes, when inserting a new node, we must update the next pointer of the previous node and the prev pointer of the next node to keep the list connected correctly.

intermediate

What happens if you try to insert a node at a position greater than the length of the doubly linked list?

The insertion should fail or be handled gracefully because the position is invalid. Alternatively, the node can be inserted at the end if the implementation allows it.

beginner

In Go, what is the typical way to represent a node in a doubly linked list?

A struct with three fields: data (any type), next (*Node), and prev (*Node), where next and prev are pointers to the next and previous nodes respectively.

intermediate

What is the time complexity of inserting a node at a specific position in a doubly linked list?

O(n) because you may need to traverse up to n nodes to reach the desired position before insertion.

Which pointers must be updated when inserting a new node at position 3 in a doubly linked list?

A Only the next pointer of node 3

B The next pointer of node 2 and the prev pointer of node 3

C Only the prev pointer of node 2

D The next pointer of node 3 and the prev pointer of node 2

When inserting at position 3, the new node goes between nodes 2 and 3. So, node 2's next pointer and node 3's prev pointer must be updated to point to the new node.

What is the first step when inserting a node at position 1 in a doubly linked list?

A Update the head pointer to the new node

B Update the tail pointer

C Find the node at position 2

D Traverse to the last node

Inserting at position 1 means inserting at the head. The head pointer must be updated to the new node.

If a doubly linked list is empty, what happens when you insert a node at position 1?

A The new node is ignored

B Insertion fails

C The new node becomes the head and tail

D The list remains empty

When the list is empty, inserting at position 1 makes the new node both the head and tail.

What is the role of the prev pointer in a doubly linked list?

A Points to the head

B Points to the next node

C Stores the data

D Points to the previous node

The prev pointer points to the previous node, allowing backward traversal.

Which of these is NOT a valid position to insert a node in a doubly linked list of length 4?

A Position 0

B Position 1

C Position 4

D Position 5

Positions start at 1. Position 0 is invalid.

Explain step-by-step how to insert a new node at a specific position in a doubly linked list.

Think about how pointers connect nodes before and after insertion.

Describe how the pointers change when inserting a node in the middle of a doubly linked list.

Focus on the four pointer updates needed to keep the list connected.

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