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

Recall & Review

beginner

What is the main idea behind inserting a node at a specific middle position in a linked list?

You find the node currently at that position or just before it, then adjust pointers to insert the new node without breaking the list.

beginner

In C, which pointer operation is crucial when inserting a node in the middle of a singly linked list?

Changing the 'next' pointer of the node before the insertion point to point to the new node, and the new node's 'next' to the next node.

intermediate

Why do we need to check if the position is valid before inserting in the middle of a linked list?

To avoid errors like accessing NULL pointers or inserting outside the list bounds, which can crash the program.

beginner

What happens if you try to insert at position 1 in a linked list?

You insert at the head, so the new node becomes the first node and points to the old first node.

beginner

How do you traverse a linked list to reach the node before the insertion point?

Start from the head and move forward node by node until you reach the (position - 1)th node.

What pointer adjustment is needed to insert a new node at position 3 in a singly linked list?

A Set new node's next to node at position 3, and node at position 2's next to new node

B Set new node's next to node at position 2, and node at position 3's next to new node

C Set new node's next to NULL, and node at position 3's next to new node

D Set new node's next to head, and head to new node

To insert at position 3, new node points to current node 3, and node 2 points to new node.

If a linked list has 5 nodes, what happens if you try to insert at position 7?

A Insertion happens at the end

B Insertion fails due to invalid position

C Insertion happens at position 5

D Insertion happens at position 1

Position 7 is beyond list length + 1, so insertion is invalid.

Which of these is the correct way to traverse to the node before the insertion point?

A Move (position) times from head

B Move (position + 1) times from head

C Move (position - 1) times from head

D Move (position - 2) times from head

You move (position - 1) times to reach the node before insertion.

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

A Check if the position is valid

B Traverse to the insertion point

C Create the new node

D Set new node's next to NULL

Always check if the position is valid before insertion.

Inserting at position 1 in a linked list means:

A Insert at the end

B Invalid operation

C Insert in the middle

D Insert at the head

Position 1 means inserting at the head of the list.

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

Think about how to move through the list and change links.

Describe how pointer changes maintain the linked list structure when inserting a node in the middle.

Focus on how links connect nodes before and after insertion.

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 exactly at the given position counting from the head

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

D. Replacing the node at the given position with a new node

Solution

1. Step 1: Understand the position concept

   Inserting at a specific position means counting nodes from the head and placing the new node exactly there.

2. Step 2: Differentiate from other insertions

   Insertion at start or end is special cases; here we insert in the middle without removing nodes.

3. Final Answer:

   Adding a new node exactly at the given position counting from the head -> Option B

4. Quick Check:

   Insert at position = Insert node at that place [OK]

Hint: Insert at position means count nodes and place new node there [OK]

Common Mistakes:

• Confusing insertion with replacement
• Thinking insertion only happens at start or end
• Ignoring the position count from head

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

easy

A. Traverse to node 2, set newNode->next = current->next; current->next = newNode;

B. Set newNode->next = head; head = newNode;

C. Traverse to node 3, set current->next = newNode; newNode->next = NULL;

D. Set newNode->next = NULL; current->next = newNode;

Solution

1. Step 1: Identify correct traversal

   To insert at position 3, we must reach node 2 (one before position 3).

2. Step 2: Link new node correctly

   Set newNode->next to current->next, then current->next to newNode to insert properly.

3. Final Answer:

   Traverse to node 2, set newNode->next = current->next; current->next = newNode; -> Option A

4. Quick Check:

   Insert at pos 3 = link after node 2 [OK]

Hint: Insert after (position-1)th node by adjusting pointers [OK]

Common Mistakes:

• Inserting at wrong node position
• Not updating newNode->next before linking
• Setting newNode->next to NULL incorrectly

3. What will be the linked list after inserting a node with value 99 at position 3 in the list 10 -> 20 -> 30 -> 40 -> NULL?

medium

A. 99 -> 10 -> 20 -> 30 -> 40 -> NULL

B. 10 -> 20 -> 30 -> 99 -> 40 -> NULL

C. 10 -> 20 -> 30 -> 40 -> 99 -> NULL

D. 10 -> 20 -> 99 -> 30 -> 40 -> NULL

Solution

1. Step 1: Identify position 3 in the list

   Positions: 1=10, 2=20, 3=30, so new node goes before 30.

2. Step 2: Insert new node at position 3

   New node with 99 points to 30, node 20 points to new node.

3. Final Answer:

   10 -> 20 -> 99 -> 30 -> 40 -> NULL -> Option D

4. Quick Check:

   Insert at pos 3 places 99 before 30 [OK]

Hint: Insert at pos 3 means new node is third in list [OK]

Common Mistakes:

• Inserting after instead of at position
• Appending at end instead of middle
• Confusing node values and positions

4. Given the following code snippet to insert at position 2, what is the error?

void insertAtPos(Node** head, int data, int pos) {
  Node* newNode = malloc(sizeof(Node));
  newNode->data = data;
  newNode->next = NULL;
  Node* temp = *head;
  for(int i = 1; i < pos - 1; i++) {
    temp = temp->next;
  }
  newNode->next = temp->next;
  temp->next = newNode;
}

medium

A. Memory for newNode is not allocated

B. newNode->next is not set before linking

C. Loop runs one time too many causing temp to be NULL

D. Head pointer is not updated after insertion

Solution

1. Step 1: Analyze loop for position 2

   Loop should run from i=1 to i < pos-1 to stop at node before insertion point.

2. Step 2: Check temp usage after loop

   Original loop runs one time too many, moving temp to node 2 instead of node 1, causing incorrect insertion or NULL access.

3. Final Answer:

   Loop runs one time too many causing temp to be NULL -> Option C

4. Quick Check:

   Loop overshoots node before position [OK]

Hint: Loop should stop at position-1 to avoid NULL [OK]

Common Mistakes:

• Loop runs to pos instead of pos-1
• Not checking for NULL before accessing temp->next
• Forgetting to allocate memory for new node

5. You have a singly linked list with 5 nodes: 1 -> 2 -> 3 -> 4 -> 5 -> NULL. You want to insert a node with value 99 at position 6. What should happen and what will be the final list?

hard

A. Node 99 is appended at the end; list becomes 1 -> 2 -> 3 -> 4 -> 5 -> 99 -> NULL

B. Node 99 is inserted at position 5; list becomes 1 -> 2 -> 3 -> 4 -> 99 -> 5 -> NULL

C. Node 99 replaces node 5; list becomes 1 -> 2 -> 3 -> 4 -> 99 -> NULL

D. Insertion fails because position 6 is invalid; list remains unchanged

Solution

1. Step 1: Check list length and position

   List has 5 nodes; position 6 means insert after last node (append).

2. Step 2: Insert node at end

   Since position is one more than length, node 99 is added after node 5.

3. Final Answer:

   Node 99 is appended at the end; list becomes 1 -> 2 -> 3 -> 4 -> 5 -> 99 -> NULL -> Option A

4. Quick Check:

   Insert at pos > length means append [OK]

Hint: Position beyond length means append at end [OK]

Common Mistakes:

• Rejecting valid append position
• Replacing last node instead of appending
• Inserting at wrong position due to off-by-one