DSA interview question

Insert a Node in a Linked List (at the Head and at a Position)

Easy · Data structures · Topics: linkedlist

The problem

The music player's "play next" button puts a song at the front of the queue, and "insert at position k" drops a song into a specific slot. With an array, both shift every later element. With a singly linked list, inserting at the head is O(1): create a node whose next is the old head, and it becomes the new head. Inserting at position k needs a walk to the node just before position k, then two pointer assignments. The order of those two assignments is where bugs hide.

Example: list 2 -> 3, insert 1 at the head → 1 -> 2 -> 3.

Function signature

Node* insertAtHead(Node* head, int x);             // returns the new head
Node* insertAtPosition(Node* head, int pos, int x); // pos is 1-indexed, 1 <= pos <= length + 1

Test cases

Input	Output list	Explanation
`insertAtHead(2 -> 3, 1)`	`1 -> 2 -> 3`	The new node becomes the head
`insertAtHead(empty, 5)`	`5`	Inserting into an empty list
`insertAtHead(7, 7)`	`7 -> 7`
`insertAtPosition(1 -> 2 -> 4, 3, 3)`	`1 -> 2 -> 3 -> 4`	In the middle
`insertAtPosition(1 -> 2, 1, 0)`	`0 -> 1 -> 2`	Position 1 is the head
`insertAtPosition(1 -> 2, 3, 9)`	`1 -> 2 -> 9`	Position length + 1 is the tail

(Confirm: positions are 1-indexed and valid; the function returns the (possibly new) head.)

How interviewers phrase it

"Insert a node at the beginning of a linked list" (GeeksforGeeks).
"Insert at the end," "insert at position k," "insert before a given value."
Warm-ups that check you can manipulate pointers without losing the rest of the list.

Clarifying questions to ask

Is the position 0-indexed or 1-indexed? What if it's out of range?
Should the function return the new head? (It must, for head insertions.)
Is a tail pointer available? (Makes tail insertion O(1).)

Follow-ups to expect

"Why must the new node's next be set before changing the predecessor?"
"Why return the head?"
"How would a dummy head help?"
"What's the cost of inserting at the tail?"
"Complexity?"

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