Insert at Middle Specific Position in DSA Go - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does 'Insert at Middle Specific Position' mean in a linked list?

It means adding a new node at a given position inside the list, not at the start or end, shifting other nodes accordingly.

beginner

What is the first step to insert a node at a specific middle position in a singly linked list?

Traverse the list to reach the node just before the desired position where the new node will be inserted.

beginner

Why do we need to update the 'next' pointer of the new node and the previous node during insertion?

To link the new node correctly into the list, so the previous node points to the new node, and the new node points to the next node.

intermediate

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

The insertion should fail or add the node at the end, depending on implementation, because the position is invalid.

beginner

In Go, what data structure is commonly used to represent a singly linked list node?

A struct with a value field and a pointer to the next node, for example: type Node struct { value int; next *Node }.

What is the correct way to insert a node at position 3 in a singly linked list?

A Traverse to node 4, set new node's next to node 3, then set node 4's next to new node

B Traverse to node 3, set new node's next to node 2, then set node 3's next to new node

C Set new node's next to head, then set head to new node

D Traverse to node 2, set new node's next to node 3, then set node 2's next to new node

To insert at position 3, you stop at node 2, link the new node to node 3, and then link node 2 to the new node.

If a linked list has 5 nodes, what happens if you insert at position 6?

A Insertion at position 6 is invalid and should fail

B Node is inserted at the start

C Node is inserted at the end (after node 5)

D Node replaces node 5

Position 6 means after the last node, so insertion happens at the end.

What is the time complexity of inserting at a specific middle position in a singly linked list?

A O(n)

B O(1)

C O(log n)

D O(n^2)

You must traverse the list to the position, which takes O(n) time.

Which pointer do you update first when inserting a new node in the middle?

A New node's next pointer

B Previous node's next pointer

C Head pointer

D Tail pointer

First set the new node's next to the next node, then update the previous node's next to the new node.

In Go, how do you define a node struct for a singly linked list?

A type Node struct { value int; next Node }

B type Node struct { value int; next *Node }

C type Node struct { value int; next interface{} }

D type Node struct { value int; next int }

The next pointer must be a pointer to Node, so *Node is correct.

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

Think about how pointers change to keep the list connected.

What are common edge cases to consider when inserting at a middle position in a linked list?

Consider positions outside the normal range and empty lists.

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