Insert at Middle Specific Position in DSA C# - 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, but somewhere in the middle.

beginner

Why do we need to traverse the list before inserting at a middle position?

Because we must find the node just before the desired position to link the new node correctly.

beginner

What happens if the position to insert is 1 in a singly linked list?

The new node becomes the new head of the list, and its next points to the old head.

intermediate

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

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

beginner

What should you check before inserting at a specific position in a linked list?

Check if the position is valid (not less than 1 and not greater than list length + 1).

In a singly linked list, to insert at position 3, you must:

A Traverse to node 3 and replace it

B Insert at the head

C Traverse to node 2 and adjust pointers

D Insert at the tail

You must reach the node before the insertion point (node 2) to link the new node.

What is the first step when inserting at a middle position in a linked list?

A Create a new node

B Traverse to the desired position

C Check if the list is empty

D Delete the node at that position

You first create the new node to insert before adjusting pointers.

If you try to insert at position 0 in a linked list, what should happen?

A Insert at head

B Throw an error or ignore

C Insert at tail

D Insert at middle

Position 0 is invalid; positions start at 1.

What pointer changes when inserting a node at position 4?

A Next pointer of node 3

B Next pointer of node 4

C Head pointer

D Tail pointer

The next pointer of node 3 must point to the new node.

What is the output after inserting 99 at position 3 in list 10 -> 20 -> 30 -> 40 -> null?

A 10 -> 20 -> 30 -> 40 -> 99 -> null

B 10 -> 20 -> 30 -> 99 -> 40 -> null

C 99 -> 10 -> 20 -> 30 -> 40 -> null

D 10 -> 20 -> 99 -> 30 -> 40 -> null

The new node 99 is inserted before the old node at position 3.

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

Think about how you link the new node between two existing nodes.

Describe what happens to the linked list pointers when inserting a node at position 2.

Focus on the connections before and after the new node.

Practice

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

easy

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

B. Adding a new element exactly between two existing nodes at a given position

C. Adding a new element only at the end of the list

D. Removing an element from the middle of the list

Solution

1. Step 1: Understand linked list insertion

   Inserting at a specific position means placing a new node between existing nodes, not just at start or end.

2. Step 2: Define middle position insertion

   Middle insertion targets a position inside the list, not at boundaries.

3. Final Answer:

   Adding a new element exactly between two existing nodes at a given position -> Option B

4. Quick Check:

   Middle insertion = between nodes [OK]

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

Common Mistakes:

• Confusing middle insertion with adding at start
• Thinking middle insertion removes nodes
• Assuming insertion only happens at end

2. Which of the following C# code snippets correctly inserts a new node at position 3 in a singly linked list?

easy

A. Node current = head; for(int i=1; i<2; i++) current = current.next; newNode.next = current.next; current.next = newNode;

B. Node current = head; for(int i=1; i<3; i++) current = current.next; newNode.next = current.next; current.next = newNode;

C. Node current = head; for(int i=0; i<3; i++) current = current.next; newNode.next = current.next; current.next = newNode;

D. Node current = head; for(int i=1; i<=3; i++) current = current.next; newNode.next = current.next; current.next = newNode;

Solution

1. Step 1: Understand zero-based vs one-based indexing

   Position 3 means after 2 nodes, so loop runs i from 1 to 2 to reach node before position 3.

2. Step 2: Check loop and insertion logic

   Node current = head; for(int i=1; i<2; i++) current = current.next; newNode.next = current.next; current.next = newNode; loops correctly to node 2, then inserts newNode after it.

3. Final Answer:

   Node current = head; for(int i=1; i<2; i++) current = current.next; newNode.next = current.next; current.next = newNode; -> Option A

4. Quick Check:

   Loop to node before position = i < position-1 [OK]

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

Common Mistakes:

• Looping too many times and skipping target node
• Using wrong loop bounds causing null reference
• Not updating next pointers correctly

3. Given the linked list: 10 -> 20 -> 30 -> 40 -> null, what is the list after inserting 25 at position 3 using this code?

Node newNode = new Node(25);
Node current = head;
for(int i = 1; i < 2; i++) current = current.next;
newNode.next = current.next;
current.next = newNode;

medium

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

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

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

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

Solution

1. Step 1: Trace loop to position before insertion

   Loop runs i=1; i<2, so current moves from 10 to 20 (node at position 2).

2. Step 2: Insert newNode after current

   newNode.next points to current.next (30), current.next points to newNode (25).

3. Final Answer:

   10 -> 20 -> 25 -> 30 -> 40 -> null -> Option D

4. Quick Check:

   Inserted 25 at position 3 correctly [OK]

Hint: Insert after node at position-1 to place at position [OK]

Common Mistakes:

• Inserting at wrong position due to loop bounds
• Not updating newNode.next correctly
• Confusing node values and positions

4. What is the error in this code snippet that tries to insert a node at position 4 in a linked list?

Node current = head;
for(int i = 1; i <= 4; i++)
  current = current.next;
newNode.next = current.next;
current.next = newNode;

medium

A. Loop runs too many times causing current to be null before insertion

B. newNode.next is not assigned before insertion

C. The loop should start from i = 0 instead of 1

D. The newNode is not created before insertion

Solution

1. Step 1: Analyze loop bounds

   Loop runs i=1 to 4 inclusive, moving current 4 times ahead, which may make current null if list length is less than 5.

2. Step 2: Identify null reference risk

   Accessing current.next after current is null causes runtime error.

3. Final Answer:

   Loop runs too many times causing current to be null before insertion -> Option A

4. Quick Check:

   Loop bounds must stop at position-1 [OK]

Hint: Loop to position-1, not position, to avoid null [OK]

Common Mistakes:

• Using <= instead of < in loop condition
• Not checking for null before accessing next
• Incorrect loop start index

5. You have a singly linked list: 5 -> 15 -> 25 -> 35 -> 45 -> null. Write the steps to insert a new node with value 30 at position 4, and what will be the final list?

hard

A. Traverse to node at position 2, link new node's next to current's next, then set current's next to new node. Result: 5 -> 15 -> 30 -> 25 -> 35 -> 45 -> null

B. Traverse to node at position 4, link new node's next to current's next, then set current's next to new node. Result: 5 -> 15 -> 25 -> 35 -> 30 -> 45 -> null

C. Traverse to node at position 3, link new node's next to current's next, then set current's next to new node. Result: 5 -> 15 -> 25 -> 30 -> 35 -> 45 -> null

D. Insert new node at head and shift all nodes. Result: 30 -> 5 -> 15 -> 25 -> 35 -> 45 -> null

Solution

1. Step 1: Traverse to node before position 4

   Move current pointer to node at position 3 (value 25).

2. Step 2: Insert new node after current

   Set newNode.next to current.next (35), then current.next to newNode (30).

3. Final Answer:

   5 -> 15 -> 25 -> 30 -> 35 -> 45 -> null -> Option C

4. Quick Check:

   Insert at position 4 means after node 3 [OK]

Hint: Insert after node at position-1 for correct placement [OK]

Common Mistakes:

• Inserting at wrong position by off-by-one error
• Not updating next pointers correctly
• Confusing position with node value