Insert at Middle Specific Position in DSA Go - Execution Trace

Concept Flow - Insert at Middle Specific Position

Start with Linked List

↓

Check if position is valid

Yes↓

Traverse to node before position

↓

Create new node

↓

Adjust pointers to insert new node

↓

Insertion complete

↓

End

This flow shows how to insert a new node at a specific middle position in a linked list by traversing to the correct spot and adjusting pointers.

Execution Sample

DSA Go

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

This Go function inserts a new node with given data at the specified position in a singly linked list.

Execution Table

Step	pos	i	current.data	Action	List State
1	3	-	1	Start at head	1 -> 2 -> 4 -> null
2	3	1	1	Move current to next	1 -> 2 -> 4 -> null
3	3	2	2	Stop loop (i=pos-1)	1 -> 2 -> 4 -> null
4	3	-	2	Create new node with data=3	1 -> 2 -> 4 -> null
5	3	-	2	Set newNode.next = current.next (4)	1 -> 2 -> 4 -> null
6	3	-	2	Set current.next = newNode	1 -> 2 -> 3 -> 4 -> null
7	3	-	-	Return head	1 -> 2 -> 3 -> 4 -> null

💡 Loop stops when i reaches pos-1 (2), then insertion happens at position 3.

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
pos	3	3	3	3	3
i	-	1	2	-	-
current.data	1	1	2	-	-
List	1 -> 2 -> 4 -> null	1 -> 2 -> 4 -> null	1 -> 2 -> 4 -> null	1 -> 2 -> 4 -> null	1 -> 2 -> 3 -> 4 -> null

Key Moments - 3 Insights

Why do we stop the loop at i = pos - 1 instead of pos?

We stop at i = pos - 1 because we need to reach the node just before the insertion point to adjust its next pointer. See execution_table row 3 where i=2 for pos=3.

What happens if the position is 1?

If pos=1, we create a new node and make it the new head by pointing its next to the old head. This is handled separately before the loop (not shown in the table).

What if the position is beyond the list length?

If current becomes nil before reaching pos-1, insertion is not done and the original list is returned unchanged. This prevents errors.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of current.data at step 3?

A 1

B 3

C 2

D 4

💡 Hint

Check the 'current.data' column at step 3 in execution_table.

At which step does the linked list first include the new node with data=3?

A Step 5

B Step 6

C Step 4

D Step 7

💡 Hint

Look at the 'List State' column to see when '3' appears in the list.

If pos was 1, what would happen to the head pointer?

A It points to the new node

B It stays the same

C It becomes nil

D It points to the last node

💡 Hint

Refer to the execution_sample code where pos == 1 case returns newNode as head.

Concept Snapshot

Insert at Middle Position in Linked List:
- Traverse to node before position (pos-1).
- Create new node with data.
- Set newNode.next = current.next.
- Set current.next = newNode.
- If pos=1, new node becomes new head.
- If pos invalid, list unchanged.

Full Transcript

This visual execution shows how to insert a new node at a specific middle position in a singly linked list. We start by checking if the position is 1, then create a new head node. Otherwise, we traverse the list to the node just before the desired position. We create a new node, link it to the next node, and update the previous node's next pointer to insert the new node. The execution table traces each step, showing variable values and list state. Key moments clarify why we stop traversal at pos-1 and how edge cases are handled. The quiz tests understanding of current node values, insertion step, and head pointer changes. This method safely inserts nodes without breaking the list structure.

Practice

1. What does inserting a node at a specific middle position in a linked list mean?

easy

A. Adding a new node only at the start of the list

B. Adding a new node only at the end of the list

C. Adding a new node exactly between two existing nodes at a given position

D. Removing a node from the middle of the list

Solution

1. Step 1: Understand linked list positions

   Positions in a linked list start from 1 at the head and increase by 1 for each node.

2. Step 2: Define middle insertion

   Inserting at a middle position means placing a new node between two existing nodes at that position.

3. Final Answer:

   Adding a new node exactly between two existing nodes at a given position -> Option C

4. Quick Check:

   Middle insertion = Insert between nodes [OK]

Hint: Middle insertion means placing node between two nodes [OK]

Common Mistakes:

• Confusing insertion with deletion
• Thinking insertion only happens at start or end
• Ignoring position indexing starting at 1

2. Which Go code snippet correctly inserts a new node at position 3 in a singly linked list?

easy

A. temp := head
for i := 1; i < 2; i++ {
temp = temp.next
}
newNode.next = temp.next
temp.next = newNode

B. temp := head
for i := 0; i < 3; i++ {
temp = temp.next
}
newNode.next = temp.next
temp.next = newNode

C. temp := head
for i := 1; i <= 3; i++ {
temp = temp.next
}
newNode.next = temp.next
temp.next = newNode

D. temp := head
for i := 1; i < 3; i++ {
temp = temp.next
}
newNode.next = temp.next
temp.next = newNode

Solution

1. Step 1: Loop to node before position 3

   To insert at position 3, loop until position 2 (i < 2) to get node before insertion point.

2. Step 2: Adjust pointers correctly

   Set newNode.next to temp.next, then temp.next to newNode to insert newNode at position 3.

3. Final Answer:

   Code snippet in option A correctly inserts at position 3 -> Option A

4. Quick Check:

   Loop to position-1, then insert [OK]

Hint: Loop to position-1 node before inserting [OK]

Common Mistakes:

• Looping too many times and skipping nodes
• Setting pointers in wrong order causing lost nodes
• Using wrong loop conditions (<= instead of <)

3. What will be the linked list after inserting value 99 at position 2 in the list 10 -> 20 -> 30 -> null using this Go code?

type Node struct {
  val  int
  next *Node
}

head := &Node{10, &Node{20, &Node{30, nil}}}
newNode := &Node{99, nil}
temp := head
for i := 1; i < 1; i++ {
  temp = temp.next
}
newNode.next = temp.next
temp.next = newNode

medium

A. 10 -> 99 -> 20 -> 30 -> null

B. 99 -> 10 -> 20 -> 30 -> null

C. 10 -> 20 -> 99 -> 30 -> null

D. 10 -> 20 -> 30 -> 99 -> null

Solution

1. Step 1: Loop runs zero times to position 1 node

   The loop condition i < 1 means no iteration; temp stays at head (value 10).

2. Step 2: Insert newNode after temp

   newNode.next points to temp.next (node with 20), then temp.next points to newNode, inserting 99 after 10.

3. Final Answer:

   10 -> 99 -> 20 -> 30 -> null -> Option A

4. Quick Check:

   Insert after position 1 = position 2 [OK]

Hint: Loop to node before position, then insert new node [OK]

Common Mistakes:

• Misunderstanding loop range causing wrong insertion
• Confusing head node position as 0 instead of 1
• Incorrect pointer assignments losing nodes

4. Identify the error in this Go code that tries to insert a node at position 4 in a linked list of length 3:

temp := head
for i := 1; i <= 4; i++ {
  temp = temp.next
}
newNode.next = temp.next
temp.next = newNode

medium

A. New node is not created before insertion

B. Pointers are assigned in wrong order causing lost nodes

C. Position 4 is invalid for a list of length 3

D. Loop runs too many times causing temp to be nil and panic

Solution

1. Step 1: Analyze loop iterations

   The loop runs from i=1 to i=4 inclusive, moving temp 4 times forward.

2. Step 2: Check list length and temp value

   List length is 3, so after 3 moves temp becomes nil; moving one more causes panic on temp.next.

3. Final Answer:

   Loop runs too many times causing temp to be nil and panic -> Option D

4. Quick Check:

   Loop beyond list length = nil pointer error [OK]

Hint: Loop only to position-1 within list length [OK]

Common Mistakes:

• Looping beyond list length causing runtime errors
• Not checking for nil before accessing next
• Confusing <= with < in loop condition

5. Given a singly linked list and a position k, write a Go function to insert a new node with value v at position k. If k is greater than the list length + 1, insert at the end. What will be the list after inserting 50 at position 5 in 10 -> 20 -> 30 -> 40 -> null?

hard

A. 10 -> 20 -> 50 -> 30 -> 40 -> null

B. 10 -> 20 -> 30 -> 40 -> 50 -> null

C. 50 -> 10 -> 20 -> 30 -> 40 -> null

D. 10 -> 20 -> 30 -> 50 -> 40 -> null

Solution

1. Step 1: Check position against list length

   List length is 4; position 5 is length+1, so insert at end.

2. Step 2: Insert new node at end

   Traverse to last node (40), set its next to new node with value 50, new node next is nil.

3. Final Answer:

   10 -> 20 -> 30 -> 40 -> 50 -> null -> Option B

4. Quick Check:

   Position > length = insert at end [OK]

Hint: If position too big, insert at list end [OK]

Common Mistakes:

• Trying to insert beyond list length without appending
• Misplacing new node in middle instead of end
• Not handling nil next pointer at list end